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Hydrology of small tributary streams in a subarctic wetland

MING-KO WOO AND PETER D. DICENZO Department of Geography, McMaster University, Hamilton, Ont., Canada L8S 4KI

Received August 15, 1988

Revision accepted February 6, 1989

Rivers traversing subarctic wetlands are fed by numerous small tributary creeks, which carry much of the wetland runoff into the rivers during the snow-free season. The wetlands, being saturated, generate abundant surface flow in the spring melt season. This amount of water cannot be accommodated by the tributary creeks and, together with general flooding of the rivers, there is little distinction between overland flow and channelled flow across the wetland, the feeder tributaries, and the major rivers. After snowmelt, the water level subsides, and most of the overland flow from the wetland is funnelled into the feeder creeks. The feeders can usually respond quickly to rainstorms and recessions are short unless surface flow is sustained by extensive marshy depressions. Along the lower reaches of the major rivers, the small tributary creeks are the main conveyors of wetland runoff to the rivers and therefore strongly modify the runoff characteristics of the rivers.

Les rivikres traversant les markcages subarctiques sont alimentkes par de nombreux petits affluents qui transportent le volume principal des eaux de ruissellement des markcages dans les rivikres, une fois la saison des neiges tenninkes. Les markcages restent saturks, assurant un dkbit de surface abondant durant la saison de fonte du printemps. Cette grande quantitk d'eau fait deborder les affluents, et une fois combin& ?i la @r ide gknkrale des crues des riv&res, il devient alors difficile de distinguer entre l'kcoulement de dkbordement et l'kcoulement chenalisk au travers les terres inondables, les affluents et les rivikres principales. Aprks la fonte des neiges, le niveau de l'eau s'abaisse, et la rnajeure partie des eaux de dkbordement sur les terres inondables se canalise dans les affluents. En gknkral, les affluents rhgissent de manikre rapide aux chutes de pluie abondantes et les pkriodes de diminution de dkbit sont courtes, B moins que l'ckoulement de surface passe par de vastes dkpressions markcageuses. Le long des segments droits des rivsres principales, l'kvacuation des eaux des terres inondables vers les riviBres se fait principalement par les petits affluents qui, par condquent, modifient wnsidtrablement les caraceris- tiques du dkbit des riv3res.

[Traduit par la revue]

Can. J . Earth Sci. 26, 1557-1566 (1989)

Introduction Rivers that drain across wetlands receive their water supply

mainly through groundwater and overland flows (Ingram 1983), but they are also fed by feeder creeks, which may be 1 - > 10 krn in length. A qualitative conceptualization of the relationship among the wetland, the feeders, and the rivers in a subarctic environment is possible, based on available field observations (Price and Woo 1988; Woo and Heron 1987). In spring, snowmelt is the major input of water to the wetland. It is quickly carried away from the wetland as overland flow and as flow in feeder creeks. Groundwater is a source of base flow but is unimportant relative to surface runoff. In summer, rainfall is the major input to most subarctic wetlands. Water is lost through evapotranspiration, or as overland and groundwater flows to the feeder creeks and streams. The feeders con- centrate much of the flow, providing a link between the wetland and its rivers.

There are few hydrological studies that concern feeder creeks explicitly. Previous research in northern wetlands have focussed upon individual hydrological processes such as surface-flow generation (Ryden 1977), streamflow and water- level responses to rainfall (Brown et al. 1968; FitzGibbon 1982), groundwater drainage in winter (Price and FitzGibbon 1982), or evaporation (Roulet and Woo 1986). Information regarding wetland water delivery by feeder creeks remains incomplete. It is the purpose of this research to study the mechanisms of water convergence and to determine the hydrological importance of feeder creeks in a subarctic wetland. Findings from this study will clarify the relationship between wetlands and their streams.

Study area The research site (51 "08'N, 79'48'W) is near the coast of

Printed in Canada 1 Imprid au Canada

southern James Bay, west of the Harricanaw River (Fig. 1). It is at the eastern end of the extensive Hudson Bay and James Bay wetland complex, sharing climatic, geologic, topo- graphic, and vegetational attributes with the rest of subarctic Ontario (Zoltai et al. 1988). Mean January and July temperatures are about -20 and 15°C respectively, with seasonal frost normally departing in mid-June and seldom returning before early September. Annual precipitation amounts to 730 mm, about 30% of which is snowfall (Environment Canada 1982). Conditions during the study period of 1985 were similar to the 30-year means for Moosonee, located 100 km west of the field site (Fig. 2).

During deglaciation, the Tyrrell Sea laid down silty clay deposits, which were subsequently overlain by sediments of silt and sand (Adshead 1983). Extensive wetlands cover the area, with belts of former beach ridges, spaced about 500 m apart, now lifted by isostatic rebound. With an extremely low regional gradient of 0.5 mlkm, the area has poor drainage, and paludification is active. The interridge marshy depressions have accumulated a greater thickness of peat. A transect across a ridge-depression sequence about 2 km inland from the coast shows ridges with a thin (0.2 m) peat layer overlying an organic, silty layer, but depressions are covered with a thick layer of peat, up to 0.75 m in the middle. This peat, with an average porosity of over 60%, is able to retain a considerable quantity of water.

Many rivers drain northward across the vast wetland. These rivers are joined by many short streams along their courses (Fig. 1). Based on seven sample reaches, the mean spacing of these feeder creeks along the river is 0.14 $ 0.04 km. One of these creeks was selected for detailed study because it has a welldeveloped drainage network and is easily accessible. The headwater of this creek is at the edges of marshy depressions

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1558 CAN. J. EARTH SCI. VOL. 26. 1989

0 Nonrecord~ng well

FIG. 1. Instrumentation at the study site, and sample reaches (A-F) of rivers showing the abundance of feeder creeks that gather water from the wetland for discharge into the rivers. R and D in bottom figure are ridge and depression sites that provided data to Fig. 5.

separated by low, drier ridges. The open marshes are covered by grasses, sedges, cattails, and short shrubs; the ridges are dominated by alder and willow or by thick stands of black spruce. The creek discharges into Washkugaw River, which drains an area of 168 krn2 but has a total relief of about 100 m. The upper part of this basin is occupied by bogs, fens, and ponds; the lower section traverses sequences of raised beach ridges and marshy depressions.

Field methods To study the relationship between the wetland, the feeder

creeks, and the wetlands rivers, various hydrological processes and linkages need to be monitored. The inputs of rainfall, snow meltwater, and lateral runoff were measured, as were outputs of evaporation, surface flows, and groundwater

flows. The storage status was also determined. In detail, different sets of meterological and hydrological instrumentation were deployed. A central meteorological site was used to gather data on air temperature (using a shielded thermistor), solar and net radiation (using Eppley and Middleton radiom- eters), ground heat flux (using a Middleton soil heat flux plate), and rainfall (using a Weathermeasure tipping-bucket rain gauge). The net radiation, air temperature, and ground heat flux records were used in the Priestley and Taylor (1972) model to compute evaporation. A snow survey, carried out at the end of the winter, using a method described by Adams (1976), provided total winter snow-storage information. Snowmelt was monitored daily at several vegetated plots by measuring the rate at which the snow-surface elevation decreased (Ah), adjusted to snow-surface density (P), so that daily melt (in mmld) was P Ah.

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WOO AND DICENZO 1559

1 of snow. Low winter temperatures in the study area indicated

APRIL MAY JUNE JULY AUGUST

FIG. 2. Comparison of solar radiation, air temperature, and precipitation during the study period with the 30-year means at Moosonee.

After the snowmelt period, continuous discharge records at three points along the feeder creek and at the lower Wash- kugaw River were obtained from stage records (recorded with Leupold - Stevens type-F recorders) using empirically derived stage - discharge rating curves. Groundwater level was recorded by Leupold - Stevens type-F recorders located at a ridge and in a marshy depression, with the wells reinforced by perforated PVC pipes. Two groundwater networks were installed to study the three-dimensional water-table fluctuation at the lower parts of the small basin. These wells were reinforced by 19 mm diameter, perforated PVC tubes covered with cloth to prevent clogging by the fine sediments. Water levels were read manually and the hydraulic conductivity at these well sites was determined using a single-auger-hole method (Luthin 1966). The tops of all the wells were surveyed with a Kern level, enabling conversion of the water depths to a common elevation.

Unchannelled surface flow was measured at seven sites by funnelling the water from a measured width of the wetland towards a central weir or towards a small opening through which the flow could be measured. The collectors were either eavestroughs or wooden sideboards, with the joints sealed by silicone. Plastic sheeting was laid behind the weirs to prevent seepage. All the collectors were oriented perpendicular to the surface flow lines, and dye tests indicated that only a negligible amount of flow bypassed the collectors.

Wetland runoff generation The major processes that directly or indirectly influence the

generation of runoff from wetlands were studied in 1985. The intensity of these processes changes during the runoff season, thereby affecting the magnitude of flow in the feeder creeks.

Snowmelt The study area was covered with different types of vegeta-

tion, each of which intercepted and trapped different amounts

little melting, so that a late winter snow survey provided an accurate estimate of snow accumulation. In both 1984 and 1985, the willow and alder bushes collected the largest amount of snow, followed by the tamarack and spruce forests. Open marsh exhibited the least accumulation because of windblown snow losses. Stream channels retained about the same amount of snow as the spruce forest; because of their sheltered nature, they gather more showdrift than do the open marshes. The amount and distribution of snow control the source of water supply for snowmelt runoff.

Snowmelt often occurs in April. The daily melt rates under various vegetation types are shown in Fig. 3. Melt was more rapid in the open, less rapid where there was a cover of alder and willow shrubs, and slowest in the spruce and tamarack forests. This is typical of the melt rates because much of the incident shortwave radiation was intercepted by the forest, reducing the melt energy to the underlying snow cover. Rapid melting yielded an abundant water supply to the wetland, but infiltration was retarded by the seasonal ground frost. Conse- quently, the wetland shed a large amount of its meltwater by surface runoff.

Rainfall Summer rain is frequent in the coastal James Bay area, with

probability of about -0.6 that there is rain on a given day (Fig. 4). The highest daily rainfall in 1985, observed on July 15, was 32 mm. Mean daily rainfall was 2.3, 3.9, and 4.9 mm for May, June, and July respectively. The moderate frequency of rainy days and the occasional occurrence of high rainfall events ensure abundant moisture supply to the coastal wetland.

Evaporation The wetland surfaces of the study basins remained at a high

level of saturation over the study period, and water supply was not a limiting factor to evaporation. The evaporation rate of an alder-and-willow-covered ridge site was computed. Although evaporation was not determined for other types of surface (e.g., spruce stands, ponds, or open marshes), the values obtained are representative of at least one-third of the basin and are comparable to those for the marshy sites 1 krn away (Price and Woo 1988). The results will illustrate the seasonal evaporation regime of the feeder-creek basin.

Daily evaporation was computed using the Priestley and Taylor (1972) model because of its robustness and proven use- fulness for the Canadian arctic (Rouse et al. 1977). Figure 4 shows the daily values determined for the period from May 15 to August 16. Before mid-May, wetland evaporation was considered to be insignificant because most of the area was covered with snow and the transpiring plants had not produced new leaves.

The results show that the mean evaporation for snow-free days was about 3 mmld, with a maximum and minimum of 7 and 1 mmld. The frequency distribution of daily evaporation shows that it is far less variable than daily rainfall during the summer.

Wetland water storage The status of water storage in the wetland is reflected in the

water-level changes. The water-level record for a site located on a raised beach ridge is given in Fig. 5. Daily water level was at or above the surface for over 65% of the study period, and never exceeded 0.1 m below the ground at this site. In the marsh area, water remained above the ground for an even longer duration, though the frequency distribution shown

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1560 CAN. I. EARTH SCI. VOL. 26, 1989

20 - - . . . . . . . .. .. ... ... ? . . . . . . . . 8OPdr rmmrr -

E SPRUCE SPRUCE E 60- - V

- -

- ALDER WILLOW ALDER WILLOW1

- - - - -

- 1 , , I , , l , , , l 1 1 -

WEN MARSH OPEN MARSH -

- (snowdrift) - 17

MAY JUNE JULY AUGUST 1985

RAINFALL

NONRAINY DAY

0 2 4 6 8 N) 12 14 16 I8 M 22 24 26 28 30 32 34 3638 40 1

40 - - :, . .; ..: .- .. . . . :, .

RAINFALL or EVAPORATION (mm/d) - ,- . .

FIG. 4. Relative frequencies of daily rainfall and daily evaporation 10 15 20 25 id I at the study site. Question marks indicate data missing.

APRIL, 1984 APRIL, 1985

FIG. 3. Daily snowmelt under different vegetation-cover types on the wetland. Question marks indicate missing data.

in Fig. 5 is biased by the loss of records from late June to early July.

When the water level (WL) is above the surface, any rise and fall will represent the gain or loss of wetland water storage per unit area (AS); or AS = AWL. When the water level falls below the surface, the actual change in storage is much smaller than the water-level change because AS = (AWL)(SY), where SY is the specific yield of the soil (assuming that there is little change in the specific retention of the wetland soil). The specific yield of the upper silt layer is 6 k 2.5%, and for the lower silt layer, it is 4 2.1 % , based on 24 soil samples taken from the raised beach ridge.

A comparison of storage change and evaporation at the ridge site demonstrates the relative importance of evaporation in the depletion of wetland storage. Table 1 summarizes the importance of evapotranspiration during several periods of water- table lowering when water remained above the surface. Dur- ing these times, water level fell at an average rate of 0.4 mmlh at the ridge site. Since the water was above the ground, the lowering rates represented the actual water loss due to evaporation and runoff. At this ridge site, evaporation accounted for about 50% of storage change, an amount that was lost to the wetland and was therefore not available as runoff.

Table 1 refers to the snow-free period. During spring, much of the water loss was from surface flow because evaporation was insignificant (Price and Woo 1988). The abundant supply of meltwater in spring and the frequent occurrence of summer rain ensured that the wetland storage was maintained at a high level throughout the study period. When the water supply exceeded the wetland storage capacity and evaporative loss, the excessive amount of water was shed as wetland runoff.

the present study, it suffices to focus upon the inflow that enters along the banks of the feeder creeks. In this regard, groundwater flux and overland flow into the creek were measured or computed as inflow per unit length along the stream banks.

Subsullface flow Groundwater flow occurs as Darcy flow and as pipe flow

from the banks of the feeder creeks. Darcy flow per unit stream length was obtained as

where D is thickness of the aquifer, d WL/d 1 is the hydraulic gradient, and k is the hydraulic conductivity. Mean hydraulic conductivity at the upper 0.65 m depth of the soil, at various well sites, was 3.1 x mls; there is a downward decrease in value, so that at 2 m depth, hydraulic conductivity is reduced by 2 orders of magnitude. The effective thickness of the aquifer is about 1 m.

The hydraulic gradient was obtained from the water-table positions along the banks. At high water level, the water table followed the surface topography, with a groundwater mound formed beneath the levees. Water then flowed both towards the creek and backward into the marshy depressions (Fig. 6). The mound diminished upstream where the levees were indistinct. When the water table declined, the groundwater mound disappeared, but the hydraulic gradient towards the creek steepened, increasing the flux rate to the channel.

Besides Darcy flow, there was flow along small pipes developed in the silt layer. Pipes are common in wetlands (Ingram 1983), and at the study area they vary in size from 0.02 to 0.1 m in diameter. Pipes are efficient conveyors of water from the wetland to the creeks. The pipes themselves are mainly fed by overland flow behind the levees, and emerge along the streambank to disharge either continuously or ephemerally into the feeder creek (Woo and diCenzo 1988).

Sullface runoff Water delivery from wetland to feeder creeks At the end of winter, most parts of the feeder creeks are

Flat, marshy terrain allows the drainage area of the feeder filled with river ice and icing and drifting snow, making it dif- creeks to expand and contract rapidly as the water level in the ficult to distinguish the channel from its surrounding wetland wetland rises or falls. It is hard to define the drainage bound- surface. Snowmelt often slushes the snow cover before surface ary (which is usually required by drainage basin studies). For flow is generated. At the study site, melting up to April 22,

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WOO AND DICENZO

--- Marshy Depression Site

RIDGE SlTE

2.95 h n

E MARSHY DEPRESSION SlTE u

W I- 5

cn GROUND SURFACE - - - - - - - - - - --

J NOTE The frequency d~strtbut~on IS b~ased

W by the loss of records dur~ng the dry spell of

CT W June -July

2 . 8 5 ] l , , I , , , , 0 2 4 6 8 1 0 1 2 1 4

FREQUENCY (dl FREQUENCY (dl FIG. 5. Water-level fluctuations at a ridge and a marshy depression site.

TABLE 1 . Storage changes and evaporative losses from a ridge site in the wetland during selected periods in 1985 when the water level was

above the ground

Change in storage Evaporation Percentage loss Date (mm) (rnm) by evaporation

June 2-7 - 32 18 58 June 13-16 -28 14 50 June 18 - 25 - 32 21 66 June 25 - 27 - 24 13 54 July 30 -

August 3 - 36 17 47

1985, resulted in 0.15-0.20 m of basal slush beneath the basin snow cover. Frozen ground hindered infiltration. Dye injected into snow trenches dug parallel and perpendicular to the creek revealed no observable flow, suggesting that the meltwater was retained within the snow cover.

Figure 7 shows the observed sequence of surface runoff initiation. During this period, only the direction of flow could be mapped, and occasionally, because the surface runoff pattern changed continually. Sporadic measurements of velocity or discharge were made where possible. These data served as

rough indicators of flow conditions. Flow began from the upper reach of the creek on April 22, and by mid-morning, discharges at Q1 and Q2 were 0.02 and 0.01 m3/s. A downstream decrease in runoff was due to the seepage of water into the snow.

Melt continued through the night and by the morning of April 23, water was flowing along the direction of the creek (0.22 m3/s at 43) to the confluence with its tributary. Runoff then followed an artificial channel dug in snow on the previous day, yielding a measured discharge of 0.19 m3/s at Q4. Below this point, water that flowed in the snow channel fanned out to merge with the backwater from the Washkugaw River. Bv late afternoon. water from outside of the feeder-creek basin ciossed the divide formed by a low raised beach, reaching the head of the feeder creek at a velocity of 0.23 m/s (Q5). Lateral flow into the creek was also observed, but was measured at only one location (Q6). This flow, at 0.12 m/s, was the first appearance of local water input, as until then streamflow was fed entirely from outside of the basin.

On April 24, backwater from Washkugaw moved up the creek with considerable velocity, and water also continued to enter the upper reaches of the creek from the marsh beyond the basin divide. Surface flow then swept across the wetland in a direction perpendicular to the length of the creek. The

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CAN. J. EARTH SCI. VOL. 26, 1989

At confluence with Washkugaw River 50 m upstream from confluence

m m -.-- _--- --__..__

Ground Surface ---- May 16, 1985

- - - -- - - - - May 19, 1985 -.-.- i. a

June 25, 1985 :a \

. . . . . . . . . July 16. 1985

0 25 50 m - CREEK "\ 100 rn upstream from confluence

m

2D-I CREEK

150 m upstream from confluence m

CREEK

CREEK

FIG. 6. Position of water table along the lower course of the feeder creek during various periods in 1985

channel was filled with chest-high water, and some melt-holes developed in the channel ice. The flow of Washkugaw River was seriously impeded by snow and ice jams (Woo and Heron 1987) and flooding of the feeder creek and its adjacent land resulted. By April 28, water was about 0.7 m deep at the meteorological site, but showed little obvious flow. On April 30, the snow dam on the Washkugaw River disintegrated and the flood quickly receded. By May 1, the ponded water drained completely and surface flow stabilized into its summer pattern.

In the snow-free period, surface flow can be divided into four characteristic types, each of which was measured at one or two sites. It is impossible to delineate the drainage divide of the experimental plots, and the daily-flow hydrographs are expressed as flow per unit width normal to the flow path (Fig. 8). Flow at OF3 and OF5 was characterized by move- ment of surface water towards the rill head. Flow at OF4 and OF6 originated from a marshy depression, crossed a raised beach ridge, and entered the creek. Overland flow at OF7 was not channelled, and it moved over what looked like a former rill that had undergone extensive paludification and was subsequently covered by a vegetated surface. Overland flow on the ridge in the lower part of the basin (OF1 and OF2) funnelled through shallow water tracks and exhibited larger responses to rainfall. During summer, flow increased exponentially with rising water table. Runoff sustained by water from the marsh depression maintained more drawn-out recession flows, indicating that these source areas provide a steadier supply of water to the creeks than do the ridge locations.

Contribution from various sources to feeder creek It is clear that in the melt season, surface runoff dominates

and is directly responsible for the breakup of the feeder creeks. Some of the meltwater released to the creeks may not reach the main river, but is retained as slush in the channel snow cover. Furthermore, through backwater effect, wetland rivers may force their water up the creeks. Thus, the creeks provide pathways through which water is transmitted from the rivers back to the wetland. Although it is impossible to obtain discharge measurements for the melt season, the qualitative observations do serve to clarify the rapidly changing flow fields of surface runoff.

After the melt period, it is possible to measure the magnitude of various runoff generating processes, and their relative contribution to the feeder creek can be determined using a water-balance approach. Between the middle and the lower segments of the study creek, inflow and outflow were measured, and the difference in their storm runoff (AQ) was compared against the sum of the lateral water inputs from surface flow (Q,), Darcy groundwaterflow (G), pipe flow (Q,), direct rainfall on the channel (PC), and channel evaporation (E,):

[2] AQ = Q, + G + Q, + PC - Ec

Stormflows were obtained by base-flow separation from the streamflow hydrograph records (Fig. 9). The period June 8 -23 was used as an example to illustrate the relative role of different sources of lateral input. Overland flow and Darcy groundwater flux were obtained by integrating mean values over time and applied to the entire length of the channel seg-

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10 20 10 23

MAY JUNE JULY AUGUST 1985

FIG. 8. Overland flow during the snow-free period. Note that OF1 and OF2 are plotted on different vertical scales. Question marks indicate data missing.

Marshy depresson be relatively insignificant, it is plausible that overland flow from the constantly saturated marshy depressions (e.g., OF3,

Wooded raised beach ' and levees OF5) was a reliable source of base flow to the feeder creek.

Overland-flow zone In contrast, overland flow that fed the lower courses (e.g., OF1, OF2) originated from the ridges, which experienced

k D~rect~on of flow greater storage fluctuations. There, the supply to the feeder creek was flashier, but diminished more rapidly when surface

Stream channel In ' snow-free p e r ~ d storage on the ridges was depleted.

Feeder creeks in coastal wetlands

FIG. 7. Breakup sequence and surface-flow pattern along the feeder creek in 1985.

ment between the inflow and outflow stations. Precipitation was measured and evaporation computed using the Priestley and Taylor model. These point values were extended to the channel segment, taking an average channel width of 1.1 m to derive total volume of direct atmospheric water gains and losses. Pipe flow was then calculated as a residual.

Results of the water balance showed that overland flow was the overwhelming process through which water was delivered to the stream channel, accounting for about 89% of the storm runoff generated within the study reach. Pipe flow contributed about lo%, while channel precipitation accounted for 2% of stormflow. Channel evaporation loss was also insignificant (- 1 %), as was groundwater flow (about 0.1 %).

Dominance of overland flow in delivering water to the feeder creek is further demonstrated by the ratio of streamflow at the lower station versus the middle station (Fig. 10). The ratio shows a significant increase in discharge downstream whenever overland flow was active. When overland flow ceased, the ratio was close to 1 .O, indicating negligible contribution from other sources.

A third station near the head of the feeder creek provided additional data to enable the computation of discharge ratio between the middle and the upper sites (Fig. 10). Unlike the ratio for lower versus middle site, this ratio remained steady at about 1.3, regardless of streamflow fluctuations during and between rainstorms. Since subsurface flow has been found to

To establish the hydrological importance of the study creek relative to other feeder creeks on the region, half of the 46 feeders along the lower 4 krn of the Washkugaw River were surveyed. On August 11, 1985, the flow from all the larger creeks (distinguished by gentle gradient and flat-bottomed channel floor) was measured, but the smaller ones were sam- pled randomly. On the basis of this survey, four types of feeders were distinguished. The small rill-like feeders contributed little runoff, yielding 0.4 x lop4 f 0.1 x lop4 m3/s, representing about 1 % of the total input from all the feeder creeks. Larger gullies constituted half of the feeders in the study area, but they accounted for only about 7% of total feeder input (mean flow was 2 x m3/s). Extensively developed gullies with gentle gradients constituted the next group of feeders. They are relatively few in number (seven of them), but contributed about 6% of the total input (flow was 5.3 x m3/s). Finally, there are six large feeder creeks with well-developed channel banks and beds, and well-inte- grated drainage network (such as the creek being studied). They provided about 86% of the total feeder input (96 X lop4 f 41 x m3/s). Thus, feeders like the one studied are the most important in delivering water from the wetland to the main rivers.

Flow characteristics of feeders and main rivers This paper has demonstrated that groundwater flow in sum-

mer is several orders of magnitude lower than overland flow and thus will not contribute much to the wetland river runoff. The presence of low levees along the river banks prevents overland flow from discharging directly into the rivers, but

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1564 CAN. J. EARTH SCI. VOL. 26, 1989

2 o.15-I Lower s a t "

A 2 0.05

- 5

E " 0 3 9 Middle Station

W 0.10

V) 0 . 0 5 5

STORMFLOW ----- 0 BASE FLOW

7 8 9 10 11 12 13 14 15 16 17 18 19 21 23 24

JUNE 1985

FIG. 9. Rainfall, evaporation, and streamflow at the input and outflow points of a stream segment. Refer to Fig. 8 for overland flow during this period.

2.0 LOWER-MIDDLE STATION I

r MAY JUNE JULY 1985

Slreamflow gauguq

r Ouer!a6d-flow measurement srte

FIG. 10. Ratios of discharge between the lower, middle, and upper sites of the feeder creek, compared with overland flow at OF2.

surface runoff from the wetland is funnelled towards the feeder creeks, which then carry the water to the rivers. The exception is in spring, when the levels of the rivers and the wetland are high and the feeders then play a minor role.

Since the feeder creeks are responsible for delivering most of the water to the main rivers, the patterns of hydrograph

MAY JULY. 'AUGUST 1985

FIG. 11. Comparison of hydrographs of feeder creek and Wash- kugaw River.

responses are expected to be transmitted to the major rivers. A comparison of the runoff characteristics of the feeder creek and the main rivers at the study site reveals their close relationship (Fig. 11). In terms of runoff regime, the Washkugaw River discharge records for 1984 and 1985 exhibit peaks in late April in response to snowmelt. Qualitative observations of many small feeder creeks during these two years indicate that their annual peak flow also occurred in this season, though much of their runoff was unchannelled.

Postmelt runoff responses to rainfall were flashy for both the feeders and the main rivers. Table 2 summarizes the range of lag times for a number of storm events. Here, lag time is considered to be the interval between the centroid of rainstorm and the hydrograph peak time. In general, runoff from the smaller basins exhibits shorter lag time, with the Washkugaw time being four times longer than those of the feeder creek. Shorter streams have their outlets nearer the source of runoff and faster hydrograph responses follow.

Hydrograph recession coefficients are also shown in Table 2. Here, the inverse of b is the recession coefficient, and b is defined by

where Q(t) and Qo are discharge at time t and at time 0 after - - the peak flow. Then, b = -[ln Q(t) - In Qo]lt, with the overbars denoting the mean values. On the whole, smaller streams recede faster than large rivers, and the feeder creek has smaller recession constants than the Washkugaw River. On the other hand, despite its limited size, the feeder creek has much longer recession times than have other streams in the subarctic and the arctic. This is due to the ability of its wetlands to sustain recession flow for a longer time, and reflects an important attribute of the wetland streamflow regime (Woo 1988). In this regard, the feeder creek is more comparable to an arctic wetland river near Point Barrow, Alaska (Brown et al. 1968), than to other subarctic streams in basins with a low percentage of wetland (Chacho and Bredthauer 1983; Dingman 1973; Sellars 1973).

Conclusions

Feeder creeks are common in subarctic coastal wetlands. They are densely spaced along the main river courses, which are often lined with low levees that prevent the direct entry of

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TABLE 2. Response characteristics of selected subarctic and arctic streams in North America

Area Absolute Response timea Recession Watershed (km2) relief (m) Surface (h) coefficientsb (h) Reference

"Twisty" Cr., 6.6 658 Bare slopes, grass, moss 1 5.2 Sellars (1973) McKenzie Mtns., N.W.T.

Glenn Cr., 1.8 237 Spruce - moss, spruce - birch 1-2 19.6-76.9, Dingman (1973) near Fairbanks, AK mean = 39

Ogotoruk Cr., Alaska 98 84 Eriophorum - Carex tundra 3-10 43 Likes (1966)

Small drainage basin at 1.6 - Marshy tundra, many ponds 2.5 -21.7 50-160 Brownetal.(1968) Barrow, AK

Washkugaw R. 168 100 Wetland 8-36 196 This study

Feeder creek to - < 10 Wetland 3 -7 75 This study Washkugaw R.

"Lag time from centroid of rainstorm to hydrograph peak. bThe value of llb in [3].

overland flow to the trunk streams. By breaching the levees, the feeder creeks provide pathways along which water from the wetlands can be efficiently conveyed to the rivers.

In winter, feeder creeks are dry because of the intense freez- ing climate. In spring, they are fed by meltwater from the wetlands as well as by backwater that moves up the rivers into which the feeders normally drain. Thus, during these times the feeders merely behave like other parts of the wetland, inter- acting with the rivers through overland flow rather than through channelled flow.

After the spring flood, the drainage pattern settles into its normal mode. Summer flow in the feeders is maintained mainly by surface runoff from the wetland. Since overland flow production is related to the water-level rises in the marsh, discharge of the feeder creeks is dictated by all the water- balance components of the wetland, including rainfall, evapotranspiration, and short-term storage changes. In this regard, the higher ground in the wetland, such as the raised beach ridges, is a less reliable supplier of water to the feeders than are the depressions in the marsh, which tend to be saturated for a longkr time each summer.

The considerable number of feeder creeks along the major wetland water way suggests that they are important in affecting the delivery of wetland water to the major rivers during the snow-free seasons. The hydrological responses of these rivers to rainfall must be strongly influenced by the flow patterns of the feeders. Hydrograph peaks in the main channels of the drainage network are delayed by the concentration time of the various sources of inflow. The main rivers have longer hydrograph response lag times partly because of channel storage and partly because they integrate the flow regimes of their various feeder creeks.

Acknowledgments Financial support for this study was provided by the Natural

Sciences and Engineering Research Council of Canada and a Northern Training grant from the Department of Indian and Northern Affairs, Canada.

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