The Hydrology of Alaskan Wetlands, U.S.A.: A Review

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The Hydrology of Alaskan Wetlands, U.S.A.: A ReviewAuthor(s): Jesse Ford and Barbara L. BedfordSource: Arctic and Alpine Research, Vol. 19, No. 3 (Aug., 1987), pp. 209-229Published by: INSTAAR, University of ColoradoStable URL: http://www.jstor.org/stable/1551357 .

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Arctic and Alpine Research, Vol. 19, No. 3, 1987, pp. 209-229

THE HYDROLOGY OF ALASKAN WETLANDS, U.S.A.: A REVIEW

JESSE FORD* Ecosystems Research Center, Corson Hall, Cornell University

Ithaca, New York 14853, U.S.A.

BARBARA L. BEDFORD Ecosystems Research Center, Corson Hall and Section of Ecology and Systematics

Cornell University, Ithaca, New York 14853, U.S.A.

ABSTRACT

Alaska's wetland resources are vast and the literature dealing directly with any given aspect of Alaskan wetland hydrology is sparse. This review focuses on hydrological function and pays particular attention to (1) hydrologic inputs to and outputs from wetlands, and (2) the influence of wetlands on peak flow regulation. The influence of several characteristic high- latitude phenomena (permafrost, glaciers, and seasonal stream icings) on the overall water balance and the volume, areal distribution, rate, and timing of water release are discussed. We find that (1) measured annual precipitation exceeds calculated annual evapotranspiration in most areas of the state, creating conditions suitable for the development of ombrotrophic wetlands; (2) during snowmelt, which is the principal event in the annual hydrologic cycle in most areas of the state, wetland soils typically have high ice contents, and thus probably do not contribute significantly to either flood storage or groundwater recharge, and (3) the existence of permafrost is critical to the existence and functioning of much of the areal extent of wetlands in the state. The implications of the state of our current understanding of Alaskan wetland hydrology for wetlands management are discussed.

INTRODUCTION

Nothing is more central to determining the existence and characteristic properties of wetland ecosystems than the hydrologic regime (Gosselink and Turner, 1978). Despite this basic fact, the hydrology of the tremendous resource embodied in Alaska's wetlands is remarkably unexplored. The vast and remote nature of these wetlands has constrained research within Alaska, and the diversity and distinctness of Alaskan wetlands from temperate wetlands have prevented easy extrapolation of principles developed elsewhere.

The geographic extent and enormous diversity of wet-

*Present address: Environmental Research Laboratory, U.S. Environmental Protection Agency, 200 SW 35th St., Corvallis, Oregon 97333, U.S.A.

land systems within the state simply overwhelm current understanding. The political boundaries of the state define a large and physiographically complex area with few major population centers (Figure 1). More than half the area of Alaska is mapped as wetland (approximately 81 million of 155 million ha), twice the total areal extent of wetlands in the rest of the continental United States. Alaskan wetlands span 20? of latitude and 58? of longi- tude, and vary in origin, substrate, and ecosystem properties. They occur in maritime, severe continental, and polar Arctic climates, are developed on glaciated and unglaciated terrain, and include both coastal and freshwater environments. Alaskan wetlands are influenced by all the factors that interact to produce wetlands in more south- erly parts of North America, as well as by characteristi-

?1987, Regents of the University of Colorado J. FORD AND B. L. BEDFORD / 209



cally high-latitude phenomena such as glaciers and permafrost. Some regions of the state are tectonically active, and the creation of new wetlands due to land subsidence associated with seismic activities is not uncommon. Gla- cial flooding and isostatic rebound following glacial wasting occur in many parts of the state and affect wetlands. Finally, Alaskan wetlands, unlike wetlands of the lower forty-eight states, are affected by a pronounced seasonal asymmetry in incident radiation, with extremely high intensity of photosynthetically active radiation during the growing season. Both seasonal asymmetry and intensity increase with increasing latitude.

Little is known about the hydrologic behavior of arctic and subarctic wetlands (Roulet and Woo, 1986), including those that occur in Alaska. The majority of studies that do exist on Alaskan wetland hydrology address either the Arctic Coastal Plain of the North Slope of Alaska, the site of major oil and natural gas interests, or the discontinuous permafrost areas in the unglaciated interior near Fairbanks. There is a paucity of information on other extensive and unique wetland ecosystems, such as the complex oxbow and meander alluvial systems asso-

ciated with glacial rivers of the interior. No published hydrologic information exists for many of the large and important wetland areas of the state (e.g., Yukon Flats, the Yukon-Kuskokwim delta, the Copper River delta, the Bristol Bay lowlands, the Kenai lowlands, the Alaska Peninsula, and southeastern Alaska).

Hydrological research in Alaska has focused on a few well-studied areas on the Arctic Coastal Plain and in the Interior. Studies on the North Slope near Barrow, Alaska, began appearing in the 1960s (Brown et al., 1968), and were extended by the work of the International Biologi- cal Programme (IBP) (Dingman et al., 1980). Some hydrologic work, including characterization of the aquifers and a 4-yr study of rainfall-runoff relations, was done in the northwestern section of the Arctic Foothills in connection with a nuclear excavation project in the Cape Thompson area (Likes, 1966; Piper, 1966; Waller, 1966). Other early hydrologic studies focused on undis- turbed upland areas near Fairbanks where wetlands develop in response to permafrost-impeded drainage (Ellsworth and Davenport, 1915; Dingman, 1971, 1973a; Dingman et al., 1971; Slaughter, 1971; Haugen et al.,

1740 170? 166? 162? 158? 154? 150' 146? 142? 138' 134? 130?

FIGURE 1. Generalized map of Alaska emphasizing geographic features.

210 / ARCTIC AND ALPINE RESEARCH



1982). The influence of south-coastal, temperate glaciers on flood events near Seward and Juneau was studied by Humphrey et al. (1983). In the interior, Anderson (1970) and Dingman et al. (1971) studied the contribution of glacial wasting to runoff. A general discussion of the relationships among vegetation, permafrost, and water relations in the saturated alluvial lowlands of the Upper Kuskokwim River valley was given in an early classic paper by Drury (1956).

Dingman's (1973b) extensive annotated bibliography of the water balance in arctic and subarctic regions ex- plicitly addresses hydrologic issues as they are influenced by features specific to high-latitude/high-altitude systems. The influence of permafrost (characteristic of most of northern and interior Alaska) on hydrology has been studied at sites in interior Alaska (Dingman, 1973a; Kane et al., 1978; Slaughter and Kane, 1979; Kane, 1980; Kane et al., 1981; Haugen et al., 1982; Kane and Stein, 1983a; Slaughter et al., 1983) and on the North Slope (Drage et al., 1983). The relationship between permafrost and groundwater in Alaska has been discussed by Hopkins et al. (1955), Williams and Waller (1966), Williams (1970), Church (1974), and Williams and van Everdingen (1973). Williams (1965) and Dingman (1975) also produced annotated bibliographies of the subject. Tolstikhin (1973) provided an English-language review of the nature and distribution of groundwater resources in permafrost regions of the USSR. Wetland recharge by subpermafrost groundwater has been studied by Cederstrom (1963) and Kane and Slaughter (1973).

Development pressures in Alaska have emphasized the need for hydrologic information regarding wetlands. Development of the Alaskan petroleum resource has resulted in larger and more densely populated urban areas, which in turn have resulted in many large industry-related and public service projects. These include, for example, the construction of the trans-Alaska pipeline and its associated roads, the creation of a transmission corridor link- ing Fairbanks with Anchorage, and several proposed large hydroelectric projects. The unique feature of these projects is their large scale juxtaposed against the fact that they have been or will be imposed on largely pris- tine areas, many of which contain wetlands. On the Arc- tic Coastal Plain hydrologic impacts of construction activities associated with the oil industry involve alterations of surface flow, leading to rapid changes in the thermal regime of permafrost-related wetlands (Pamplin, pers. comm., 1986). Despite this, few studies of postconstruc- tion hydrologic impacts have appeared in the literature (Pamplin, pers. comm., 1986, but see Pamplin [1979] and Klinger et al. [1983]). In adjacent regions of Canada, oil and gas exploration in the Mackenzie River delta and

other parts of the Northwest Territories has produced several hydrologic studies (Anderson, 1973, 1974; Ander- son and MacKay, 1973, 1974; MacKay et al., 1973; Sellars, 1973; Newbury, 1974; Outhet, 1974; Soulis and Reid, 1978; Roulet and Woo, 1986) that may be appli- cable to similar Alaskan systems.

On a more local scale, recent urban expansion has put pressure on municipal wetlands. In Anchorage, a limited land base has resulted in significant development of several small urban wetland areas (Anchorage Planning Dept., 1982). Hydrologic investigations at three of these wetlands were undertaken to guide development projects (Mosquito Lake wetlands: Dowl Engineers, 1983; Klatt and Connors Bogs: Anchorage Planning Dept., 1982; Glass, 1986a, 1986b). Hydrologic data are also available for uplands in the Point MacKenzie region across Cook Inlet from Anchorage (Glass, 1983). The area immedi- ately to the west is a large peatland that could be subject to direct hydrologic impacts from the extensive defores- tation now taking place on these wetlands. In Juneau, hydrologic investigations are currently being initiated in the wetlands of the Mendenhall Valley (Adamus, 1986; Siegel, in press).

In this paper we focus primarily on two hydrologic functions as they relate to Alaskan wetlands: (1) hydrologic inputs to and outputs from wetlands, and (2) flood storage and desynchronization (i.e., peak flow regulation). As with wetlands elsewhere, the factors that influence these functions include local climate, basin mor- phometry, the ratio of watershed area to wetland volume, and the material properties of the peats and their under- lying mineral substrates. In the arctic and subarctic climates found in Alaska, however, water is present in solid phase for much of the year, and the influence of such typical high-latitude phenomena as glaciers, permafrost, and naleds (massive winter icings that occur in river valleys) on the volume, areal distribution, rate, and timing of water release become critical in understanding the hydrology of these high-latitude wetlands. We explore these issues by considering the various components of the overall water balance as they are affected by these and other factors, by analyzing the influence of wetlands on the hydrologic response of watersheds, and by discussing the pertinent groundwater literature. In large part, we invoke first principles and our own understanding of wetland ecosystems to draw relevant conclusions from a literature that does not address wetland hydrology directly. We conclude by discussing the implications of current understanding for using generalized criteria to evaluate Alaskan wetlands for regulatory or management purposes.

THE WATER BALANCE

BACKGROUND AND RELATION TO WETLANDS The water balance is a fundamental way of characteriz-

ing the steady-state hydrologic cycle of a region. Ingram (1983) has suggested a general water-balance equation of the form:

J. FORD AND B. L. BEDFORD / 211



P + N-D-ET-AW-n = 0 (1)

where

P = precipitation N = total surface and subsurface recharge to the

wetland, including seepage, pipe flow, diffuse surface flow, and open channel flow

D = total surface and subsurface discharge from the wetland, including the same elements as N

ET = evapotranspiration (evaporation + transpiration) A W = change in water storage

n = error.

Precipitation (P) and evaporation are meteorologic functions, but evapotranspiration (ET) can also be affected strongly by the transpiration of vascular plants, as well as by various physical characteristics of the soil itself, such as soil texture, depth, and color. Recharge (N) and discharge (D) are landscape functions. They also include a meteorologically driven component in areas of glacial storage. Water storage (W) may include permafrost and glaciers in addition to factors shared with systems in temperate regions, such as groundwater storage, soil moisture, lakes, and biomass (Dingman, 1973b).

Ingram uses the term "recharge" (N) to refer to inputs to the wetland from other systems and "discharge" (D) to refer to outputs from the wetland. This usage differs from standard terminology in the literature pertaining to wetland functions and values, in which the focus is on wetlands as sources of recharge to the groundwater. We will use the terms "recharge" and "discharge" in the more traditional sense (e.g., Viessman et al., 1977) to refer to recharge or to discharge from the groundwater system.

The quantity P-ET, net evaporative water balance, affects wetland development (Moore and Bellamy, 1974; Damman, 1979). When P-ET is negative, a net deficit occurs in atmospherically supplied water. In these cases, wetlands can exist only in areas of recharge from the surrounding landscape, i.e., telluric recharge. Because telluric recharge is typically richer in electrolytes (including calcium) than meteorologic recharge, the resulting minerotrophic wetlands (fens) usually are distinguished easily on the basis of water chemistry from the ombrotrophic wetlands (bogs) that develop in areas whose principal supply of water is from the atmosphere (Gorham, 1956; Gorham et al., 1985). Fens can exist under any P- ET regime as long as there is some source of telluric recharge. Bogs, however, cannot continue as active vegetation units in areas with a net evaporative deficit. In regions that have experienced changes in the P-ET balance over long time periods, bogs will begin either to expand in area (increasingly positive P-ET) or to dry and decompose (increasingly negative P-ET), producing characteristic weathering horizons.

Carefully instrumented water balance studies for arctic and subarctic wetlands are almost nonexistent. Recent work by Woo and his associates in the Northwest Terri-

tories (e.g., Roulet and Woo, 1986) are a notable exception.

NET ANNUAL EVAPORATIVE WATER BALANCE In order to emphasize the potential for wetland devel-

opment in the Alaskan landscape, we have constructed a map of net annual evaporative water balance (Figure 2). The only previous map of net annual evaporative water balance for Alaska is extremely generalized (Newman and Branton, 1972). Our more detailed map (Figure 2) has been developed from maps of potential evapotranspiration (Patric and Black, 1968) and precipitation (Hartman and Johnson, 1978), but is useful only for the lowland and coastal regions represented by the monitoring networks. Even in lowland areas, however, precipitation estimates can be questionable, especially in regions like the North Slope, where total precipitation is low, wind plays a major role, and most of the total precipitation occurs as snow (Benson, 1982). For these reasons, Figure 2 should be considered only as a general guideline. Four major factors, which we discuss below, affect the accuracy of Figure 2 in any specific area: (1) the accuracy of precipitation measurements, (2) the accuracy of evapotranspiration estimates, (3) the influence of mountainous topography on both precipitation inputs and evapotranspiration rates, and (4) the influence of frequent fog and cloud cover, especially at coastal sites.

(1) Precipitation estimates may be in error (Black, 1954; Anderson and MacKay, 1974; Dingman et al., 1980; Benson, 1982; Sturges, 1986). Biases towards systematic underestimation of precipitation are well known (e.g., Hare and Hay, 1971) and have been confirmed for far northern environments by Cook (1960), Findlay (1969), and Woo et al. (1983). Studies at Barrow indicate that National Weather Service gauges underestimate water equivalents of the winter snowpack by a factor of 1.6 to 3.0, probably due to wind effects on snow gauges (Ding- man et al., 1980; Benson, 1982). Because a significant part of the annual meteorologic input of water occurs as snow (over 50%7o at Barrow over the period 1941 to 1970 [Dingman et al., 1980]; about 30% at Glenn Creek near Fairbanks over the period 1952 to 1967 [Dingman, 1971]), the large errors associated with snowpack measurement are particularly significant and will strongly affect net evaporative water balance calculations.

Total summer precipitation is also underestimated at Barrow by a factor of at least 1.1 (Dingman et al., 1980), and possibly much more (Benson, 1982), due to failure to measure trace and occult (fog, dew) precipitation (Dingman et al., 1980). Benson (1982) found that 43 to 69%7o of all daily summer precipitation measurements at Barrow during 1972 to 1979 were recorded as "Trace," which gives an erroneous total of zero precipitation when summed over all these days. Similarly, 55 to 76% of all daily winter snow measurements were recorded as "Trace." Black (1954) had earlier attributed discrepancies in summer precipitation measurements to wind effects, but studies by Brown et al. (1968) found these discrepan-




cies to be uncorrected by the installation of windshields. On the other hand, wind effects on rainfall measurements in the Mackenzie Delta region decreased measured precipitation by at least 15%7o (Anderson and MacKay, 1974).

(2) Good estimates of evapotranspiration are difficult to obtain and infrequently made for Alaska. The most widely used estimate is Thornthwaite's potential evapotranspiration (PET), which is calculated from temperature data and provides an upper limit for water losses to the atmosphere. The few tests of Thornthwaite's PET in Alaska show good agreement between measured and calculated PET (Patric and Black, 1968). Surprisingly, there is also good agreement between estimated PETand simple Class A pan evaporation data (Table 1), probably because in regions dominated by tundra, transpiration losses account for only 7 to 20%o of total evapotranspiration (Dingman et al., 1980). In these regions water losses from

moist vegetated surfaces are about equal to losses from open water (Dingman et al., 1980), making actual rates of evapotranspiration similar to simple evaporation rates.

Evapotranspiration can also be estimated as the difference between precipitation and runoff. For the few Alaskan sites in which these estimates have been compared to PET and/or pan evaporation data, serious discrepancies occur between this procedure and the others (Table 1). Underestimates of precipitation and overesti- mates of runoff for large rivers, such as the Colville, may account for the discrepancies (Dingman et al., 1980).

(3) In mountainous regions, higher precipitation inputs and lower rates of evapotranspiration will elevate the net evaporative water balance. Patric and Black (1968) point out that permanently snowcapped, extensively glaciated mountainous regions are evidence of a cold, wet, high-elevation climate that, for lack of monitoring information,

FIGuRE 2. Generalized map of net annual evaporative water balance for Alaska (precipitation minus evapotranspiration). This figure was constructed by overlaying maps of calculated PET (Patric and Black, 1968) on precipitation maps (Hartman and Johnson, 1978) and calculating the difference. Both precipitation measurements (250 stations) and calculated PET (315 stations) are generally from low- lying coastal and river valley areas, and are not valid for higher elevations. Because precipitation is in fact often seriously underestimated (see text), the value of the upper bound of precipitation in each region was used. The upper bound of calculated PET was also used. PET varies over a smaller range than precipitation (20-61 cm yr-1 vs. 13-610 cm yr-1), and PET increments are small relative to precipitation increments (5 cm yr-~ vs. 13-305 cm yr-1). Therefore, the statewide pattern of net evaporative water balance resembles the precipitation map for the less arid regions of the state.




TABLE 1 Comparison of evapotranspiration estimates

Thornthwaite Area PET (mm)

Glenn Creek Glenn Creek Barrow Mackenzie Delta,

Canada

465 356

N.C.b

335

Class A pan Precip. minus evap. (mm)a runoff (mm)

482 365 160

290C

152 194 60

75

Season

Annual Thaw Thaw

Reference

Dingman (1971) Dingman (1966) Brown et al. (1968)

Annual Anderson and MacKay (1974) Anderson (1974)

aUncorrected for pan coefficient. bN.C. -Not calculated. cMeasured with 500-cm3 black porous disc atmometers; includes estimated snow sublimation of 40 mm.

is not covered by their PET map. Precipitation inputs even in the relatively arid Brooks Range can be higher than average values reported by Hartman and Johnson (1978) for surrounding lowland areas (Kilday, 1974; Wendler et al., 1974).

(4) Frequent fog and cloud cover at coastal sites will depress thaw season evapotranspiration rates, particularly at high-latitude stations (Dingman et al., 1980).

Our map shows that calculated net annual evaporative water balance is close to zero over much of the North Slope and ranges from slightly negative to strongly positive over the lowlands of most of the state (Figure 2). However, the regions with negative P-ET are not completely arid. For long-term steady-state conditions, P-ET is approximately equal to runoff (RO), which is >0 on the Arctic Coastal Plain. The negative values we calcu- late for this region are probably due to (1) underestimation of P, (2) overestimation of PET, and/or (3) PET>ET Because of the limitations inherent in our methods, it is likely that the true net annual evaporative water balance is more positive than calculated over much of the state, despite the fact that summer PET balances summer precipitation at several individual stations throughout the state (Sanderson, 1950; Patric and Black, 1968; Luthin and Guymon, 1974; Braley, 1980; Kane and Stein, 1983a; Gieck et al., 1983).

Our estimates of net annual evaporative balance differ greatly from those of Newman and Branton (1972), who drew a P-ET= 0 isopleth that roughly follows the curve of the Alaska Range. This difference is significant, because our estimates considerably extend the region over which development of ombrotrophic wetland is theoreti- cally possible. The development of other kinds of wetlands will be determined by water balance as well as topography, type of surficial deposits, permeability of the substrate (including permafrost), timing and duration of flooding, groundwater configuration, and coastal influences.

INFILTRATION Groundwater recharge, runoff, and, to some extent,

flood storage are controlled by infiltration rates, which

CURVE B

t^ CURCURVE C

z ,\ -CURVE D

z

TIME >

FIGURE 3. Simplified schematic diagram of infiltration rates in frozen soils (from Gray [1970], by permission of National Re- search Council of Canada). Curve A demonstrates infiltration characteristics when the soil is frozen when saturated, or when an impervious ice layer develops on the surface during the melting period. Under these conditions, infiltration rates are low and consequently the runoff coefficient is high. Curve B demonstrates infiltration characteristics when a soil is frozen at a high moisture content (70-8007o field capacity). In this case, some meltwater is able to penetrate the soil and transfer heat to melt the ice-filled pores. As the soil warms and the pore ice melts, the infiltration rate increases. In time, the intake rate will again decrease due to the increasing soil moisture. Curve C demonstrates infiltration characteristics when the soil is frozen at a low moisture content and the soil temperature at the time of snowmelt is near freezing. In this case, only the small pores are filled with ice, and these are thawed rapidly with the down- ward movement of water. Under these conditions infiltration proceeds as under normal unfrozen conditions. Curve D demonstrates infiltration characteristics when the soil is frozen at a low moisture content but the soil temperature at the time of snowmelt is well below freezing. Water entering the soil is frozen in the pores and movement is inhibited. Note that this con- ceptual diagram is very generalized and does not include abso- lute units. Infiltration rates in these four scenarios can vary by orders of magnitude and do not necessarily attain approximately similar values after some period of time. More detailed information on infiltration rates into soils of differing composition frozen at different water moisture contents can be found in Kane and Stein (1983b).




in turn depend on type of substrate, presence or absence of permafrost in the catchment, and general basin mor- phometry. Permafrost is widespread throughout Alaska and acts to retard infiltration and limit groundwater recharge. It is primarily the presence of permafrost in the North Slope and the eastern part of the Mackenzie River delta region, N.W.T., Canada, that permits the paradoxi- cal development of wetlands even under relatively arid conditions (Brandon, 1966).

The amount of ice in frozen soils strongly affects infiltration rates. Infiltration rates are significantly lower in fully and partially saturated frozen soils relative to soils frozen at low moisture contents, as indicated schemati- cally in Figure 3. Direct studies of snowmelt infiltration show that frozen wetland soils with high ice content have substantially lower infiltration rates than frozen dry soils (Kane and Stein, 1983a, 1983b).

Glacially derived alluvial silts are also widely dis- tributed throughout Alaska and are associated both with riparian wetlands and with floodplain wetland complexes originally derived from major river systems (Ford and Bedford, unpublished). These glacially derived silts gen-

erally tend to retard infiltration relative to peats (e.g., Boelter, 1965; Roulet and Woo, 1986); however, frozen silts with low moisture content can readily accept snowmelt at rates greater than the snow can melt (Kane and Stein, 1983b).

PERMAFROST Permafrost (Figure 4) can be considered "a confining

bed of low but finite permeability" (Williams and van Everdingen, 1973). Many Alaskan wetlands are affected by permafrost, including those in most of the northern and interior parts of the state. Alaskan wetlands that are not usually influenced by permafrost include those in southeastern Alaska, the Kenai peninsula, the lower floodplains of the Susitna and Copper River valleys, and the southern coastal marshes (including large parts of the Bristol Bay region) (Hopkins et al., 1955).

Because permafrost represents a phase change of water, it affects all aspects of the hydrologic cycle. Conse- quently, any factor that affects the thermal equilibrium will affect hydrologic processes. Thermokarst features, in which there is settling of the land surface due to melt-

Generally underlain by continuous permafrost

m Underlain by discontinuous permafrost

1i.1Uderlan by isolated 1 mass of permafrost

Generally free from permafrost

GLENN CREEK AND -CARIBOU-POKER CREEK EXPERIMENTAL WATERSHEDS

0 250 560km

FIGURE 4. Research areas with hydrologic data superimposed on a map of continuous, discontinuous, and sporadic permafrost areas of Alaska (permafrost map is redrawn from the permafrost map included on the Joint Federal-State Land Use Planning Commission for Alaska's [1973] map of major ecosystems of Alaska).




ing of ground ice, may develop in response to disruption of the thermal equilibrium, whether by regional climatic change or anthropogenic activities (e.g., Mackay, 1970; French and Egginton, 1973). Along large river systems, bank erosion can be particularly severe in permafrost terrain because of large-scale sloughing associated with thermal erosion and structural weakness due to ice wedges and other kinds of ground ice (Outhet, 1974), or ice lenses that are remnants of the previous winter's stream icings (Kane, 1981).

The precise characteristics of the permafrost layer will be determined by its structure (e.g., honeycomb vs. con- crete frost) and by its large-scale configuration. Infiltra- tion rates and groundwater recharge are usually low when permafrost is present. One exception occurs in limestone regions, especially where subsurface karst drainage systems predate the onset of permafrost and permit rapid infiltration along predefined drainage channels (Brook, 1983).

Small-watershed studies have shown that permafrost strongly affects summer runoff after storms. Basin storage is decreased and surface runoff is increased (Ding- man, 1971; Slaughter and Kane, 1979; Haugen et al., 1982; Gieck et al., 1983; Slaughter et al., 1983). In discontinuous permafrost regions, aspect can determine the presence or absence of permafrost, which in turn determines the distribution of wetlands (Dingman and Koutz, 1974). Flood hydrographs for catchments underlain by permafrost tend to be flashier and more responsive than those from permafrost-free catchments (Slaughter and Kane, 1979). Peak flows and associated suspended sediment yields are higher in permafrost-dominated basins, and base flows are lower than in permafrost-free basins (Slaughter et al., 1983).

Permafrost also affects snowmelt runoff. Kane et al. (1978) applied a paired-watershed approach to study the effects of permafrost on snowmelt-runoff generation in the discontinuous permafrost zone near Fairbanks. Dur- ing snowmelt, permafrost-free soils contributed little runoff, whereas large volumes of water moved downslope through the organic mat at the permafrost-dominated site. The implication is that the permafrost-free catchment could contribute to groundwater recharge, whereas the permafrost-dominated catchment is unlikely to do so.

In permafrost regions, the short-term (seasonal) water balance may be affected by changes in storage (A W) due to seasonal thaw. However, in permafrost areas no streamflow occurs during extended dry periods, suggesting that thawing of the active layer is not a source of streamflow (Dingman, 1973a; Dingman et al., 1980).

RELATIONSHIPS BETWEEN ALASKAN WETLANDS AND GROUNDWATER RECHARGE AND DISCHARGE

Little is known about how Alaskan wetlands affect groundwater recharge or about the areal extent and geographic distribution of wetlands maintained by groundwater discharge. However, some inferences can be drawn from the literature on individual arctic and subarctic drainage basins that are known to contain wetlands.

Surface water generally plays a major role in nourish- ing Alaskan wetlands by virtue of the existence of vast floodplains coupled with direct inputs from glacial meltwater. In any particular case, however, sources and mag- nitudes of both surface water and groundwater feeding

250-

200 -

150 -

100 -

50-

O'

0 0 0

x

u

0

0

-7

-6

-5

-4

-3

-2

0

- 7 E 0 3

-6 "

-5 m

-4 m

-3 (A

-2 m x

- I 0 0 0 0 '"

250

200

150

100

50

0

250-

200-

150-

100-

APR MAY JUNE JULY AUG SEP OCT

DATE

FIGURE 5. Annual variability of seasonal discharge, lower Susitna River. Selected patterns of Susitna River discharge at Susitna Station (river mile 25), south-central Alaska. Data from U.S. Geological Survey (1977, 1981, 1982). This graph is constructed from daily discharge data; only every sixth day is represented. The Susitna River receives meltwater from several gla- cierized tributaries in the Alaska Range, including the Skwentna and Chulitna rivers. High flows at Susitna Station, near the mouth, integrate all inputs, which include spring snowmelt, summertime glacial melt and high-country rains, and late summer/early autumn rains. Winter snowpack in the Alaska Range determines the magnitude of early spring discharge, which can vary by as much as 2 x (panels (a) and (c)); summer temperatures and precipitation determine the magnitude and duration of summer discharge, and precipitation during the late summer/early autumn rainy season determines the magnitude and duration of late-season discharge.


~



high-latitude wetlands are difficult to identify and measure. In areas of low relief, catchment divides are difficult to identify because few permanent drainage networks develop (Brown et al., 1968; Drage et al., 1983), and patterns of surface water movement may be transient. In regions where the principal input of precipitation is as snow, year-to-year variability in snowpack distribution and volume will greatly affect the local snowmelt runoff (Lewkowicz and French, 1982) that is a major contribution to wetland water budgets. Artesian discharge of subpermafrost groundwater is known to occur in several regions and to be related to both lake and wetland formation in and around the Fairbanks area (Ceder- strom, 1963; Kane and Slaughter, 1973).

Spring snowmelt is the single most important event contributing to the annual water budget of northern catchments, and is typically the most significant time for groundwater recharge because of the potential for sub- stantial infiltration to occur (Church, 1974; Drage et al., 1983; Gieck et al., 1983; Kane and Stein, 1983a, 1983b). In interior Alaska, winter moisture deficits can be gen- erated during the long, cold, dry winters because surface soils contribute moisture to the overlying snowpacks (Harlan, 1974). In wetlands, if this deficit occurs, it is usually satisfied relatively quickly. Because most wetlands are underlain by frozen wet soils during snowmelt, infiltration is minimal and subsequent snowmelt typically generates runoff rather than groundwater recharge (Roulet and Woo, 1986). For instance, during the snowmelt period in the Mackenzie River delta, 85? o of the available water in the Boot Creek catchment (snowpack + precipitation) appeared as runoff (Anderson and MacKay, 1974).

Snowmelt events dominate annual stream hydrographs because snowmelt represents the volume of precipitation accumulated over more than half the year. At Barrow, where the landscape is dominated by wetlands and thaw lakes, more than 50% of the annual flow volume is discharged in a few days after snowmelt despite the low relief (2 to 5 m) (Dingman et al., 1980). Farther to the east along

the Arctic Coastal Plain, more than 75%o of the annual discharge of the Kuparuk River occurs during June snowmelt (Drage et al., 1983). Seasonal hydrographs for six streams in the Brooks Range show that 50%o of the flow over the period of record (which did not include late summer and autumn) was discharged in the first 30 d of the runoff season (Onesti and Walti, 1983). The entire total annual surface discharge from a small wetland underlain by permafrost near Baker Lake, Northwest Ter- ritories, occurred over a 21-d period following snowmelt (Roulet and Woo, 1986). The total subsurface discharge from this wetland for the entire 1983 study period was exceeded by 2 h of surface water flow during peak snowmelt runoff (Roulet and Woo, 1986). In the largely unvegetated Jason's Creek basin (Devon Island), which is underlain by permafrost, 75% of the annual discharge left the catchment within less than 3 wk of the onset of snowmelt (McCann and Cogley, 1972). Farther south in the discontinuous permafrost region of the Caribou- Poker creeks watershed, annual hydrographs also are typically dominated by spring runoff, except during years of light winter snows and heavy summer rains (Haugen et al., 1982).

In the large northern rivers that have their headwaters in mountainous regions, the timing of peak flows is not restricted to the spring snowmelt period. Heavy summer rainfalls add to continuous base-flow contributions by glacial melt and snowmelt from the upper catchment, with occasionally spectacular results (e.g., MacKay et al., 1973). Riparian wetlands associated with large northern rivers may be above field capacity at any time during the open water season, depending on rainfall patterns and snow and glacial wasting in distant source areas. This situation occurs, for instance, in the lower Susitna River Valley, where episodes of maximum discharge in the lower Susitna and consequent patterns of areal flooding in the associated Flathorn Lake wetland complex can occur at any time from June to September (Ford and Bedford, unpublished) (Figures 5 and 6).

HYDROLOGIC RESPONSE

The hydrologic response of a watershed (Table 2) can be characterized by (1) the relationship between the amount of incoming precipitation (P) and the amount of storm runoff (RO) it generates (RO/P ratios), (2) the lag time between precipitation inputs and peak storm discharge, and (3) the length of time until normal baseflow conditions are reestablished, i.e., recession time. Wetlands influence these properties of a watershed, and thereby affect floodwater storage and regulation of peak flows. Again, data specific to Alaskan wetlands are sparse and inferences must be drawn from other watershed studies.

RUNOFF/PRECIPITATION (RO/P) RATIOS High RO/P ratios are found in areas with good topo-

graphic relief, well-defined surface drainage networks,

and impermeable substrates. In catchments underlain by discontinuous permafrost near Fairbanks, ratios for individual storms range between 3 and 205%7o with catchment means of 18 to 49%7o (Dingman, 1971; Haugen et al., 1982; Table 2). At Ogotoruk Creek, which is underlain by shallow continuous permafrost, short-term studies show that RO/P ratios range from 40 to > 100?o (probably due to release of other water held in storage as well as to errors in RO measurements), and that peak runoff rates for storms of long duration approach the rate of precipitation (Likes, 1966). Ratios of RO/P in excess of 100%0o probably reflect underestimates of total precipitation (D. Kane, pers. comm., 1986; Dingman, pers. comm., 1987), as well as the arbitrary nature of hydrograph separation techniques, which is in any case a general problem in esti-




>X nkm

HIGH WATER STAGE LOW WATER STAGE 20 JULY 1980 4 SEPTEMBER 1980

FIGURE 6. High and low water stages of the Flathorn Lake/Susitna River wetland system, south-central Alaska. Lake level is related to river stage. High discharge in the Susitna River is accompanied by increased subsurface flow into the wetlands along an old meander, as well as by direct overtopping of the natural levee between the river and the wetlands. At high water stages, the outlet temporarily becomes an inlet as Susitna River water backs up into the lake.

AUGUST 1978

mating runoff from individual storms (Dingman, pers. comm., 1987).

For any given storm, RO/P ratios for individual catchments will vary with the extent of permafrost in the basin. Slaughter and Kane (1979) analyzed individual storm hydrographs for a northeast-facing permafrost-dominated catchment (C-3) and a south-facing permafrost-free catchment (C-2) at Caribou-Poker Creeks Research Watershed (Figure 7). The permafrost-dominated catchment had peak flows 3 to 4 times as high as those of the permafrost-free catchment and had much longer recession limbs. The long recession times probably result from detention storage by the Sphagnum moss-black spruce vegetation typical of soils underlain by permafrost (also see "Groundwater"). Later studies (Slaughter et al., 1983) confirmed the consistency of these results, and docu- mented much higher suspended sediment loads exported from the flashy C-3 catchment in high water years.

Several interesting points emerge from a comparison made by Haugen et al. (1982), who plotted cumulative frequency distributions for instantaneous discharge measurements on permafrost-free (C-2) and permafrost- dominated watersheds (C-3) and plotted them together with a similar number of measurements from Dingman's (1971) permafrost-dominated Glenn Creek watershed (Figure 8). First, the two permafrost-dominated catchments, C-3 and Glenn Creek, have higher peak flows and lower base flows than the permafrost-free site (C-2). Gen- erally speaking, the Glenn Creek site behaves similarly to C-3, even though significant differences in relief ratios and drainage densities exist between them. Permafrost- free catchment C-2, on the other hand, behaves quite dif- ferently than either of the other two catchments, even though the median discharge values of C-2 and C-3 are similar and the two catchments are well-matched on sev-

SEPTEMBER 1978 10 20 30

C_

z 0

i-L

0 w 0w

0-

0) -J.

4-

0

w

u

0

FE

o

-

a-

<)

U-)

-J lL

z

l-

Z

~E

10 20 30 10 20 30

AUGUST 1978 SEPTEMBER 1978

FIGURE 7. Precipitation and streamflow for permafrost- dominated and permafrost-free catchments, Caribou-Poker Creeks Experimental Watersheds, interior Alaska (redrawn from Slaughter and Kane [1979]). Watershed C-3 contains significant permafrost deposits and is more responsive to individual precipitation events than permafrost-free watershed C-2.




eral geomorphological parameters. Catchment C-4, which is of mixed aspect and has intermediate proportions of permafrost, has intermediate streamflow patterns and flow-duration curves (Slaughter et al., 1983). These analyses suggest that the extent and distribution of permafrost strongly affect both flood storage capacity and

streamflow response. By contrast, Newbury (1974) found little difference between permafrost-free and permafrost-dominated basins when he compared published data on RO/P ratios from individual storms for three permafrost basins and two permafrost-free basins. However, at least one of the permafrost-free basins is underlain by

TABLE 2 Hydrologic response of selected watersheds in Alaska and Northwest Territories, Canada

Max. unit Catchment area % of

area discharge snowmelt Area Vegetation (km2) Permafrost (m3 s-~ km-2) as runoff

Peter Lake, Mackenzie Delta, N.W.T.a Tundra 46.0 Continuous 0.05k N.D.

Boot Creek, Mackenzie Delta, N.W.T.a Taiga 31.0 Continuous 0.22k 85%o

Barrow, Alaskab,c 75% in Carex aquatilis 1.57 Continuous N.D. N.D. Glenn Creek, Alaskade Varies with aspect 1.8 Discontinuous- N.D. N.D.

47%7o in black spruce- 60% of catchment Sphagnum

8% in alder-willow-sedge Caribou Creek, Alaskaf Varies with aspect-wooded 23.8 Discontinuous 0.12k N.D. Poker Creek, Alaskaf Varies with aspect-wooded 59.8 Discontinuous 0.09k N.D. Ogotoruk Creek, Alaskag 39% in Eriophorum tussock 97.1 Continuous N.D. N.D.

and 13% in Eriophorum- Carex wet meadowh

Colville River, Alaskai N.D. Continuous 0.12 N.D.

Event (runoff/ Unit area precip) ratio Lag time Recession time recession times

Area (%) (h) (h) (h km-2)

Peter Lake, Mackenzie <5%7o; increases in autumn N.D. N.D. N.D.

Delta, N.W.T.a Boot Creek, Mackenzie <5%o; increases in

autumn N.D. 61-105 2 - 3.4 Delta, N.W.T.a

Barrow, Alaskab,c 5?% in dry seasons; >11 dry seasons; 50-160 31.8-101.9 15-63% in wet seasons 6.5-7.0 wet seasons

Glenn Creek, Alaskade 3-42%,e 2-13%d (mean 0-9e (mean of 27 20- 80e 11 - 43e of 16 storms= 18o)f storms = 2. )e

Caribou Creek, Alaskaf 9-79% (mean of 20 N.D. "Varies widely"i N.D. storms = 35 %)

Poker Creek, Alaskaf 7-205% (mean of 13 N.D. N.D. N.D. storms = 49%)

Ogotoruk Creek, 40-> 100%o 3-10 "Quite variable"g "Quite variable"g Alaskag but about 35i but about 0.36i

Colville River, Alaskai N.D. N.D. N.D. N.D. General comments Strongly dependent on Variable source area Generally longer at Values for 40 streams

antecedent condi- concepts apply; high latitudes due in the conterminous tionsb,d,e stream response time to low rates of U.S. range from

independent of evapotranspirationde 0.005-0.32 h km-21 watershed wetnessd,e

aAnderson and MacKay (1974). bBrown et al. (1968).

cBrown et al. (1980). dChaco and Bredthauer (1983). eDingman (1971).

fHaugen et al. (1982). gLikes (1966).

hJohnson et al. (1966). iArnborg et al. (1966). JDingman (1973a).

kAt snowmelt. 'Holtan and Overton (1963) as cited by Dingman (1973a).

mN.D. -no data.




dense lacustrine clays, and would be expected to behave similarly to the permafrost basins.

In areas of low relief where low P-ET permits drying of the active (seasonally thawed) layer and provides potential storage capacity, midsummer storms may contribute little to runoff. RO/P ratios for midsummer storms on the order of 5%7o or less have been reported for both Barrow, Alaska (Brown et al., 1968) and the Mac- kenzie River delta, N.W.T. (Anderson and MacKay, 1974) (Table 2). The actual fate of the precipitation received in any given event, however, depends strongly on the antecedent moisture content of the catchment. For example, in 1963 the thaw season at Barrow was quite wet, and RO/P ratios ranged from 15 to 63%o (Brown et al., 1968). At Glenn Creek, RO/P ratios range from 2 to 42%o depending on the antecedent conditions (Dingman, 1971, 1973a; Chaco and Bredthauer, 1983). At the nearby Caribou-Poker Creeks watersheds, however, the relationship between RO/P ratios and antecedent discharge (a general indicator of basin wetness) was not significant at the 0.05 probability level, possibly due to lack of resolu- tion in the data base (Haugen et al., 1982). More recent studies at Glenn Creek indicate that runoff can begin before the moisture deficit of the surface organic mat is completely satisfied (Chaco and Bredthauer, 1983). Both of these results suggest that local watershed characteristics may modify the general relationship between antecedent wetness and RO/P ratios in specific instances.

In the Mackenzie River delta area, Anderson and MacKay (1974) found that low midsummer RO/P ratios are followed by a return to higher values in late summer. Storms of a magnitude that produced virtually no response in July produced significant runoff with long recession limbs in August (Figure 9). These authors attribute this pattern to a decrease in rates of evapotranspiration during the colder and wetter late summer season that enhances infiltration, melting the permafrost and increasing base flow. This conclusion departs from the findings of Dingman (1973a; Dingman et al., 1980), whose data suggest that permafrost melt does not contribute significantly to runoff. Late summer increases in responsivity are not seen at Barrow, where 30-yr temperature averages indicate similarity of mean August and mean July temperatures (Dingman et al., 1980). Because RO/P ratios depend heavily on the antecedent wetness of the watershed, Anderson and MacKay's (1974) finding may simply reflect saturation of watershed soils following the accumulated precipitation inputs between 13 July and 22 August. This possibility is consistent with Roulet and Woo's (1986) finding of small detention storage for a small, flat wetland in the continuous permafrost region of the Northwest Territories. On the other hand, other data from Barrow indicate that diurnal varia- tions in discharge do occur and appear to be related to evapotranspiration (Brown et al., 1968), which suggests that seasonal changes in basin responsivity may have a component that is influenced by changes in rates of evapotranspiration.

n , I, I

N E

Cz

E

z. U cI

0.01

0.001

0.000

(68.5% perm(

I I I

2 10 20 3040506070 80 90 98

Percentage

FIGURE 8. Cumulative frequency distributions of discharge measurements, Caribou Creek and Glenn Creek watersheds, interior Alaska (redrawn from Haugen et al. [1982], by permission of U.S. Army Corps of Engineers, Cold Regions Research and Engineering Laboratory). The permafrost-dominated catchments C-3 and Glenn Creek behave similarly to each other despite differences in geomorphology and watershed size. Permafrost-free catchment C-2 behaves in a noticeably different manner, despite close similarities in geomorphology and watershed size with catchment C-3.

LAG TIMES Lag time is the time between the center of mass of a

precipitation event and peak streamflow. Generally speaking, lag times for arctic and subarctic wetland catchments tend to be much shorter than those for their temperate counterparts, especially for rains of high intensity (Likes, 1966; Dingman, 1973a; Table 2). Because of the shallowness of the active (thawed) layer, potential storage capacity is quickly satisfied when permafrost (Fig- ure 4) is present, even where peat deposits are deep. The fine-grained, relatively impermeable nature of glacial silts that underlie many Alaskan wetlands at shallow depth can also limit storage capacity and promote rapid runoff, although porosities can be high, especially in frozen silts of low moisture content (Kane, pers. comm., 1986).

Lag times of 1 to 2 h are not unusual in small (less than a few square kilometers) watersheds. Even under the low relief conditions at Barrow, average lag time in wet years is only 6.5 to 7 h, although in drier years it can increase to over 11 h, due to greater detention storage capacity


s - - w - t - -I I



and increased subsurface flow (Brown et al., 1968). At Glenn Creek in interior Alaska no systematic relationship occurs between lag time and antecedent conditions (Dingman, 1971, 1973a) except when the watershed is extremely dry. The short lag times typically found at this site (Table 2) reflect the more common condition of near- surface saturation of soils adjacent to the channel.

RECESSION TIMES Recession times are a measure of the length of the de-

clining flow period. Streamflow recessions are commonly approximated by a simple exponential decay function (e.g., Dingman, 1971):

18-

17-

16-

15-

14-

3-

12-

10-

1- 9' U. 0 z 8- (K

q = qoe-'t (2)

where

t* = recession constant q = discharge at time t q0 = discharge at time t= 0.

Impermeable catchments with high topographic relief and little vegetation have short recession times because of the lack of detention storage capacity in their catchments (e.g., Ogotoruk Creek: Likes, 1966). Generally speaking, though, high-latitude catchments with good

973

6

4 -45 -40 - 35 -30 - 25 -20 - 15 -10

-5 0

-Q

m 0

z

Is

15 20 25 5 10 15 20 25 5 10 15 20 25 5 10 15 20 25 5 10 15 20 25 5 10 15

MAY JUNE JULY AUGUST SEPTEMBER OCTOBER

FIGURE 9. Runoff and precipitation at Boot Creek basin (31 km2), Mackenzie River delta, N.W.T. (1973) (redrawn from Anderson and MacKay [1974], by permission of Environment Canada). Note that the storms of 13 July and 22 August are of similar magnitude but produce different runoff responses (see text).


.J.I I.



vegetative cover have unit-area recession times up to 10,000 times greater than lower latitude systems (Table 2). Much of this latitudinal difference may be due to the low evapotranspiration rates characteristic of high- latitude moss-dominated systems (Dingman, 1971, 1973a). The effect of evapotranspiration on recession times has been studied by Dingman (1971, 1973a), who used a modeling approach to demonstrate that the 4 x variation in storm recession times at Glenn Creek (Table 2) can be explained by small (0.00 to 0.25 mm h-1) differences in evapotranspiration rates.

Recession times also depend on the physical characteristics of wetland peats. Little work has been done on the water-retention capabilities of Alaskan peats (but see Dingman's [1971: Appendix B] work on Sphagnum and Pleurozium schreberi mosses). Generally speaking, Sphagnum mosses have a significant storage capacity because of the presence of large hyaline cells that can rapidly gain or lose water. Thus, although both moss and sedge peats contain > 80% water by volume at saturation (Boelter, 1970), undecomposed moss peats can release 45 to 80%0o of this to drainage, whereas herbaceous peats yield only 10 to 15%70 (Walmsley, 1977; Verry and Boelter, 1978). Boelter (1965, 1972) demonstrated that the hydraulic conductivity of different peat types can vary over 5 orders of magnitude depending on botanical origin and degree of decomposition.

Permafrost thaw is unlikely to contribute substantially

to base flow or to lengthened recession times. At both Barrow and Glenn Creek, streamflow disappears during extended droughts when climatic factors should be optimal for permafrost thaw (Dingman, 1971, 1975; Dingman et al., 1980). Heat transfer by percolating rain- water also should have only minor effects on permafrost thaw, inasmuch as the thaw effects of rain falling directly on glacial ice unprotected by an insulating cover of organic soils are likely to be minimal (Huntsinger, pers. comm., 1986). The thaw effect, however, will depend upon soil ice content. Seasonally frozen soils containing little ice can thaw very rapidly with rainfall infiltration.

In summary, the relatively long recession times characteristic of high-latitude wetland catchments probably result from a combination of (1) storage and slow release by peats with high-water retention capabilities, and (2) low evapotranspiration rates (Dingman, 1973a; Church, 1974). The lower the slope and the longer the flow-path through peat, the longer the recession time will be. How- ever, the ability of a high-latitude wetland to actually attenuate streamflow response is probably small during snowmelt (Roulet and Woo, 1986). Direct studies of the storage capacity of a small wetland underlain by permafrost have demonstrated that because the peats retain a large proportion of the water released by thawing of seasonally frozen upper soils, there is little excess storage capacity for water entering the wetland from other sources (Roulet and Woo, 1986).

GROUNDWATER

The groundwater term in wetland water budgets is difficult to measure (Carter et al., 1978). It is often calculated by difference from the water balance equation (equation 1), but the large number of terms involved in each component (see, e.g., Ingram, 1983), and the diffi- culty in obtaining precise estimates for many of them, make this approach subject to serious error (Winter, 1981). Direct observations can be made using a combination of observation wells and piezometer nests in order to determine the configuration of the water table and the hydraulic potential at various locations in the wetland. This approach has been adopted in urban Anchorage in order to determine the feasibility of wetland development (e.g., Anchorage Planning Department, 1982; Dowl Engi- neers, 1983; Glass, 1983, 1986a, 1986b). Other ap- proaches used to quantify groundwater flow include studies of groundwater chemistry and river base flow. Al- though these methods have been more widely applied in Alaska, few such studies have been tied directly to wetlands.

Information on groundwater-flow systems in permafrost regions is sparse, although the pace of investigation is accelerating as development pressures encroach on high-latitude systems (e.g., Tolstikhin, 1973). Several characteristics distinguish groundwater hydrology in permafrost regions from that in other areas. First, rates

of groundwater movement are generally lower than in equivalent temperate regions because of the increased vis- cosity of water at low temperatures (1.2 to 1.8 times higher) (Brandon, 1966; Church, 1974). Second, "groundwater" in these regions includes subpermafrost, intra- permafrost, and suprapermafrost (active layer) groundwater.

In regions of deep, continuous permafrost, little or no hydraulic connection exists between the subpermafrost and suprapermafrost zones. Permafrost can act as an upper confining bed on the subpermafrost aquifer, which is typically artesian (Williams and Waller, 1966; Ceder- strom, 1963; Kane and Slaughter, 1973; and Linell, 1973). Although few direct data exist concerning water quality of subpermafrost groundwater, what there is suggests that subpermafrost groundwater can be brackish or saline (Williams and Waller, 1966; Williams and van Ever- dingen, 1973), presumably due to salts leaching from the organic or inorganic matrix. Deep unfrozen zones can occur above the permafrost table under deep rivers and lakes (Williams and Waller, 1966). Potable water in these regions is usually suprapermafrost water that accumulates in depressions in the permafrost table (Williams and Waller, 1966). This nonsaline groundwater is dependent on surface-water sources for recharge and may occur only seasonally. Nonsaline recharge of subpermafrost bedrock




aquifers is thought to occur only where streams flow across the strike of unfrozen permeable limestone and sandstone beds (Waller, 1966).

In regions of discontinuous permafrost, there may be hydraulic connections between supra- and subpermafrost groundwater through unfrozen zones (taliks) that occur under lakes, rivers, and valley bottoms, and on south- facing slopes. These taliks are often saline (e.g., Ceder- strom, 1961), and talik temperatures are therefore often below 0?C. If a thawed zone links the shallow and deep aquifers, artesian pressure can result in discharge to both wetlands and ponds. In the Fairbanks area, some wetlands (Cederstrom, 1963) and bog lakes (Cederstrom, 1963; Kane and Slaughter, 1973) have been demonstrated to be fed by subpermafrost groundwater under artesian pressure. However, this situation appears to be geographically patchy (Hartman and Carlson, 1973).

The contribution of Alaskan wetlands to groundwater recharge probably is negligible. With the exception of high-altitude and coastal systems, most Alaskan continuous and discontinuous permafrost regions are relatively arid (Kane and Stein, 1983a). Summer precipitation generally is balanced by summer evapotranspiration (Sanderson, 1950; Patric and Black, 1968; Braley, 1980; Kane and Stein, 1983a). Recharge thus depends on spring snowmelt, which in turn is affected by infiltration rates in frozen soils. However, infiltration rates in seasonally frozen wet soils are two orders of magnitude lower than in seasonally frozen dry soils (Kane and Stein, 1983a). In discontinuous permafrost regions where groundwater recharge does occur, recharge zones are primarily permafrost-free areas, with little or no contribution from permafrost-dominated wetland soils (Kane and Stein, 1983a).

In the larger northern river systems, groundwater discharge can be an important source of winter flow (Church, 1974). In many areas, however, seasonal frost can reach the permafrost table, restricting groundwater flow, increasing the hydrostatic pressure, and forcing water through cracks in the ice cover. As the continuous winter base flow rises to the surface and freezes in the frigid winter air, thick stream and river icings are created that can range in size from a few square yards to thou- sands of hectares (Dean, 1984). As shown in Figure 10, these icings (also referred to as aufeis, or naleds) serve to redistribute total water discharge from small, continuous outputs during the cold season to a single, short- duration, catastrophic output at the beginning of the warm season (Sokolov, 1973). In spring, aufeis melt can comprise a significant portion of the total annual water volume of a river (Sokolov, 1973).

E 60- / / \ E / \

/

E ' 40-

20-

0 Sept Oct Nov Dec Jan Feb Mar Apr May June July Aug ept ct

Month

volume of naled --- area of naled

Naled accumulation

S Naled depletion by runoff

FIGURE 10. Changes in volume and area of Anmancyndinskaya naled, expressed as proportions of maximum at the end of winter (redrawn from Sokolov [1973]). Note that the winter's accumulation of ice peaks in late April, as runoff begins. Run- off peaks in mid-June, and is primarily discharged in the 6-wk period from mid-May to early July.

Dean (1984) has mapped the distribution of stream icing zones in Alaska. Icings are particularly large and prevalent in and near mountainous terrain in the Brooks Range, northeastern Alaska, and the north flank of the Alaska Range, and decrease in number and size to the south. They typically occur in the same place from year to year because their creation depends on stable characteristics of local landforms that influence groundwater flow rates, such as aquifer composition and geometry. The physical mechanics of aufeis growth have been de- scribed by Kane (1981).

Groundwater discharge wetlands that border river systems (palustrine wetlands sensu Cowardin et al., 1979) might be expected to contribute to aufeis formation, because their saturated soils contribute to deep seasonal frost. However, the relationship between aufeis formation and wetland cover has never been systematically studied. Some floodplain wetlands, especially those isolated from the active floodplain by natural levees, may depend on the exceptionally high flows downstream of spring aufeis melt.

ADDITIONAL FACTORS INFLUENCING ALASKAN WETLAND HYDROLOGY

Several factors specific to arctic and subarctic environments preclude the straightforward application to Alaska of general principles governing hydrologic relations in

temperate wetlands. These factors exert significant influence over hydrologic relations and will affect wetland functions. The primary factors are (1) permafrost, which




influences the development of wetlands, (2) the movement of water between surface and subsurface reservoirs, and (3) the runoff and storage characteristics of glaciated drainage basins, which introduce significant differences in seasonal flow patterns, sediment loads, and water quality between these and nonglaciated systems. Other factors include the following:

(1) Physiographic instability. Hydrologic relationships can be rapidly shifted and redefined in Alaska's geologi- cally dynamic landscape. Topographic instability-as evi- denced by uplifts and downfaulting, subsidence, and other tectonic factors-produces an inherent variability on a temporal scale far shorter than that of temperate wetlands.

(2) Fluvial processes. The power of water to rework the conditions that govern its own movement and, therefore, the conditions that govern wetland development and hydrologic function, reaches a magnitude in many

Alaskan river systems that is seldom seen in the lower forty-eight states. Characteristic high-latitude phenomena associated with fluvial systems, such as thermokarst erosion and scouring produced by glacial outburst flooding, also play locally important roles.

(3) Aeolian inputs. Significant wind-borne inputs of fine-grained glacial silts from the exposed bars in major river systems to adjacent wetlands occur along major river valleys of the state. Over time, these continuous inputs can decrease soil permeability and infiltration rates in receptor wetlands.

(4) Climate. Extremes of climate accentuate conditions and processes that influence wetland hydrology through effects on thermal balances. Examples include thaw-lake cycles, delayed exposure of wetlands to seasonal insolation due to the persistence of avalanche runouts and drifted snow, and low transpiration rates.

CONCLUSIONS

STATUS OF EXISTING KNOWLEDGE Information on the hydrologic functions of Alaskan

wetlands is remarkably sparse, especially given the domi- nance of wetlands on this vast landscape. What information is available is geographically concentrated and usually does not directly address issues pertaining to the hydrologic functions (sensu Adamus and Stockwell, 1983) of wetlands. Most studies have been conducted either on the Arctic Coastal Plain or in the Interior near Fairbanks. Virtually nothing is known about the hydrology of major types of wetland systems, such as the alluvial wetlands of most rivers (e.g., Yukon Flats, about 65,000 km2), or the high-elevation alpine wetlands of the southeast coast. Similarly, no hydrologic information exists for extensive geographic regions of the state, including the Seward Peninsula, the Yukon-Kuskokwim delta, and the Alaska Peninsula.

Only a few generalizations about the hydrologic functions of Alaskan wetlands can be made on the basis of existing studies. These pertain to the effects of the net annual water balance and permafrost on the development and existence of wetlands, and to the probable influence of snowmelt and permafrost on runoff and groundwater recharge. First, measured annual precipitation exceeds calculated annual evapotranspiration in most areas of the state, thus creating conditions suitable for the development of ombrotrophic wetlands. In southeastern and much of coastal Alaska, strongly positive water balances are a function of high precipitation inputs, whereas in the arid northern and interior regions, water balances are controlled by the combination of low evapotranspiration rates and the presence of low-permeability permafrost, which allows the development of wetlands under semiarid (<250 mm precipitation annually) conditions.

Second, in all but coastal regions where rainfall domi- nates annual precipitation, the principal event in the annual hydrologic cycle is snowmelt. A significant proportion of spring snowmelt runs off directly to streams,

especially in permafrost regions where frozen ground inhibits infiltration. Wetland soils tend to have a high ice content during snowmelt and thus probably do not contribute significantly either to flood storage or to groundwater recharge.

Third, the importance of permafrost to the existence and functioning of wetlands cannot be overstated. In discontinuous permafrost regions, aspect can determine the presence or absence of permafrost, which in turn determines the presence or absence of wetlands. The presence of permafrost in a basin affects summer runoff by decreasing basin storage and enhancing translation of precipitation to runoff. Peak flows in permafrost-dominated catchments tend to be higher and base flows lower than in adjacent permafrost-free catchments, suggesting that overall, high-latitude wetlands do not play an important role in streamflow regulation.

IMPLICATIONS OF THE CURRENT UNDERSTANDING OF ALASKAN WETLAND HYDROLOGY FOR WETLAND EVALUATION SCHEMES

Most wetland evaluation schemes used in regulatory and management decisions, including that proposed for the Federal Highway Administration (Adamus and Stock- well, 1983), make several assumptions that are not valid for Alaska. These assumptions pertain to the availability of basic planning tools required in a hydrologic assessment process, and to the accessibility of sites for field evaluation, as well as to the status of fundamental scientific understanding of hydrologic relationships.

The availability for Alaska of planning tools as basic as large-scale topographic maps, soils maps, aerial photographs, and tide tables is not comparable to the lower forty-eight states. Although topographic maps of the 15-min series (1:62,500) do exist for most of the state, they are based on black and white photos taken between 1948 and 1952 which are of poor quality and are tectonically and culturally out of date in many parts of Alaska.




For most of the state the only soils maps are from an ex- ploratory soil survey done at a scale of 1:250,000. Aside from satellite imagery and scattered aerial photographs of particular sites, the only aerial photographs available for most of the state are a recent set of color infra-red photographs taken at a scale of 1:65,000 and a syn- chronous set of black and white photographs at a scale of 1:120,000. Maps of permafrost distribution are extremely generalized. For its 61,000 km of coast, the state has only four tide stations, none of which has data for the eighteen years recommended for a full tidal cycle. For these reasons, even the first-level assessments about wetland hydrologic functions, which might otherwise be made in an office on the basis of general understanding and these planning tools, cannot be made for Alaskan wetlands.

Further, problems associated with accessibility to sites and the expense of reaching them impose severe restric- tions on the acquisition of field data. Most of the state cannot be reached by road; wheeled planes, float planes, boats, or helicopters must be used. The distances to be traveled are large, and considerable financial and logis- tic constraints must be overcome to obtain the seasonal and annual monitoring required for hydrologic studies.

The lack of fundamental scientific understanding im- plies that generalized indicators, or criteria for predict- ing wetland hydrologic function cannot be identified with confidence for most regions and wetland types in Alaska.

Further, a great deal of research must be undertaken before such features can be identified. In the meantime, intensive site-specific studies, such as those recently undertaken for the Mendenhall Valley in southeastern Alaska (Adamus, 1986; Siegel, in press), will be required for particular projects and particular wetlands.

ACKNOWLEDGMENTS

This paper was prepared at the request of the National Wet- lands Technical Council (NWTC) as part of a program to examine the potential for regionalizing the Adamus-Stockwell (1983) method of wetlands evaluation. It was presented in May 1986 at the NWTC workshop for the Alaska region. We thank M. S. Christy, P. Krebs, L. Lodwick, M. Maurer, C. Slaugh- ter, J. Thilenius, and L. Viereck for helpful discussions; S. L. Dingman, R. Huntsinger, D. L. Kane, D. Kiefer, D. Siegel, P. Spencer, and an anonymous reviewer for careful reading of the manuscript; R. Herrick for typing; and M. Howland and J. Walters for graphics. This publication is ERC-125 of the Eco- systems Research Center (ERC), Cornell University, and was supported in part by the U.S. Environmental Protection Agency Cooperative Agreement Number CR811060. Additional funding was provided by U.S. Environmental Protection Agency grant R810150-01 to Barbara L. Bedford. The work and conclusions published herein represent the views of the authors, and do not necessarily represent the opinions, policies, or recommendations of the Environmental Protection Agency.

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Ms submitted January 1987




Documents

The Hydrology of Alaskan Wetlands, U.S.A.: A Review