Influence of physical processes on the design, functioning and

Special Issue: I lydrologic Restoration of Coastal Wetlands Wetlands Ecology and Management vol. 4 no. 2 pp 73 91 (19t)71

SPB Academic Publishing by, Amsterdam

Influence of physical processes on the design, functioning and evolution of restored tidal wetlands in California (USA)

J. Haltiner I, J.B. Zedler ~, K.E. Boyer 2, 'G.D. Williams 2 and J.C. Callaway 2 sphilip Williams & Associates, Ltd.. Pier 35, The Emharcadero. &m Francisco. CA 94133. USA. :Pac([ic Estuarine Research LaDoratorv (PERL), Biology Department, San Diego State University, San Diego, CA 92182-4625, USA

Keywords: Salt marsh, wetland hydrology, wetland restoration, mitigation

Abstract

The performance of two intertidal wetland mitigation projects constructed by the California Department of Transportation (Caltrans) in the Sweetwater Marsh National Wildlife Refuge (SMNWR) in San Diego Bay was evaluated over 5 years. Most of the Sweetwater wetland complex has been altered this century, including diking (with subsequent subsidence), filling, modification of the tidal regime, freshwater inflow and sediment fluxes. The mitigation project goals inchided a range of functional criteria intended to support two endangered bird species (light-tooted clapper rail and California least tern) and one endangered plant (salt marsh bird's-beak). While the mitigation projects have achieved some of the performance criteria established in the regulatory permits (particularly, those related to fish), vegetation criteria for one of the bird species have not been met. The initial grading (in relation to local tidal datums) should support the target plant species, but growth has been less than required. Shortcomings of the habitat include elevated soil and groundwater salinity, low nutrient levels (especially nitrogen, which is readily leached from the coarse substrate), and eroding topography (where a single oversized and overly sinuous channel and the lower-than-natural marshplain result in high velocity surtlace water flow and erosion). The failure to achieve a large plain at low-marsh elevations highlights the importance of a more complete understanding of the relationship between the site physical processes (topography, hydrology, climate, geomorphology), substrate conditions, and biotic responses.

Introduction

Wetland restoration science and practice has developed during the past 25 years in California in response to the loss or alteration of approximately 90-percent of the historic tidal wetlands in the state, and the widespread recognition of the values provided by these ecosystems. While the concepts of wetland creation, restoration and enhancement are widely viewed as a positive practice, their im- plementation has been complicated by the introduction of wetland "mitigation." The practice of creating new wetlands, or enhancing degraded sites in compensation for losses, has developed in

response to Federal and State regulation of activities that degrade or restllt in wetland loss. The stlc- cess of mitigation projects, both in complying with pemlit conditions and in replacing tile values lost, has been tile subject of extensive discussion and controversy (e.g., Race 1985, Harvey and Josse- lyn 1986, Race and Fonseca 1995).

Some of this uncertainty is linked with the relatively limited understanding of the complexity of wetland ecosystem functioning. The basic goal and success criterion t'or many wetland restoration projects involve the rapid establishment of vegetation cover. In particular, while the importance of links between the site physical processes (mor-

74

phology, hydrology, soils, and climate) and biological responses are widely acknowledged, our understanding of these relationships is relatively simplistic and our ability to engineer these processes remains even less dexeloped. The practice of wetland restoration mirrors ottr gradually evolving understanding of these relationships. The pur- pose of the present study was to evaluate the t\mc- tioning of restot'ed wetlands at a more detailed level and to link the biotic responses to the site physical processes.

History of California restoration practice

The "'first generation" of coastal wetland restoration attempts in California in the early 1970s were based primarily on tile recognition of the relationships of vegetation to elevation. During this phase it was recognized that by re-opening diked areas to tidal circulation, wetland vegetation would recolonize barren areas itl elevation zones col-- responding to tidal datums. The time for vegetation re-establishment and the potential l\~r more rapid vegetation establishment by planting were evaluated.

Based on these results, a second generation of restoration projects was developed in response to the need for larger projects and the often degraded condition of the available sites. Project types included the placement of dredge spoils (to raise low areas) or the excavation of uplands (fill, dredge. spoils, etc.) to create intertidal wetlands. One key focus was on the relationship between slough channels alld the marshplain. On the basis of the work of Myrick and Leopold (1963) and Pestrong (1965), empirical techniques using reference wetlands were dexeloped to aid in channel design. These included relationships between channel design parameters (thahveg elevation, cross-sectional area, sinuosity, and network density) and the diurnal tidal prism (linked to wetland area) at a pat +- titular site and channel location (Haltiner and Williams 1987). The optimal design elevation of the marshplain was also assessed. In settings with a hi,Ha~ sedimentation potential (for example, San Francisco Bay wetlands) it was recognized that grading the marshplain below the expected equilibrium elevation provided several benefits: 1) The

more frequent tidal inundation at the lower elevation allowed problem soils to more quickly develop acceptable salinity, pH, nutrient, etc., conditions: 2) the subsequent naturally-deposited sediments supported better vegetation establishment: and, 3) this configuration also allowed the more rapid development of lower order slough channels, (because only the primaly, or higher order channels were usually excavated).

Williams and Florsheim (1994) established these concepts for the else of dredge material placement in the design of a 130-ha restoration project on a subsided limner wetland in north San Francisco Bay. They provided t-ecommendations on the max- imum elevation of dredge material (0.5 m below Mean High High Water, or MHHW) to allow for the development of a dense channel network and the use of internal levees to limit wind fetch and prevent erosion of the evolving marshplain. Moni- toring of previously constructed projects also provided information on the importance and complexity of substrate conditions. Particle size distribution, hypersalinity, low pH and lack of nutrients have been considered limiting for vegetation establishment at various restoration sites. In addition, the time required for optimal vegetation establishnaent may vary widely. For example, in the 50-ha Muzzi Marsh restoration project in Marin County (San Francisco Bay), establishment of cordgrass was minimal for about seven years following the re-introduction of tidal circulation, and the project was considered unsuccesst\d (Race 1985). However, over a few more years, the vegetation covera,,e~ expanded rapidly and the site supported a key population of the endangered light- t\~oted clapper rail. It is now considered one of the most successful restoration projects in San Fran- cisco Bay (Williams e,'a/. I988).

Improving restoration practices

hnprovement of functional success in the next generation of wetland restoration projects requires a more detailed understanding of tile relationships between physical and biotic processes, particularly on difficult sites. In addition to basic research, a key source of data comes from an in-depth assessment of the performance of prior projects.

9 .-:,..:.

" . : ~ ~ ) ( - , ~ -

. , :...

, "~

~:. ~a~g~s .-=. ...:

npowder~-~is Point "C-- .~ '

E S t r e e t M a r s h

Y

j-__ 1

S w e e t ~ v a t e r R i v e r -" - . . . " . . ~ } ~

v i

-L 2 2! . . . . . --:------ ::::i r 77>',-:7 ;!

F'iy 1. The 1887 US('&GS map of the study area.

One of the primary criticisms of mitigation prqiect design and permitting has been the lack of clearly defined and measurable goals to judge success and project compliance. A second concern has been the lack of adequate monitoring programs to measure project peM'onnance and functional attributes. However, both detailed success criteria and provisions for monitoring were included in the development of a mitigation program in the SMNWR (Sweetwater Marsh National Wildlife Rethge) in southeastern San Diego Bay. The projects were developed by the California Department of Transportation (Caltrans) and the U.S. Army Corps of Engineers (COE) as mitigation tot wetland impacts resulting flom improvements to the adjacent freeway and construction of a flood control channel by the US Army Corps of Engineers (COE). The mitigation projects were completed in

75

1990, and monitoring of the site's biological attributes has been conducted on an annual basis. Key physical processes were monitored immediately tbllowing protect construction and five years later. In addition, several ecological experiments were conducted. Assessment of the site pert\~r- mance provides insights on both general tidal restoration project design and the complexity and level of detail attributable to a specific site or ecological conlillunity.

Site morphology

San Diego Bay was an alluvial valley draining the Otay, Sweetwater, and San Diego Rivers before the most recent episode of sea level rise (5-20,000 years BP). The post-glacial sea-lexel rise of approximately 100m inundated the valley. Tidal wetlands were formed at the mouths of the various creeks and rivers and in quiescent backwater zones. Available sediment to create and sustain the wetlands was provided by the m~\ior fluvial systems. An 1887 map of the mid-bay wetlands shows their relatively natural conditions (Figure 1 ). The north-south line and different shading suggest a distinction between wetland zones, perhaps representing the mudflat-marsh boundary, the wrack line, or difl'ering elevation or vegetation zones. The raihoad track and bridge over Paradise Creek (PC) serve as a usethl reference to identity subsequent chan,,es~ . Durin~ the next centL!.ry, the SMNWR wetlands were significantly altered as the area was urbanized. The types of changes that occurred are characteristic of degradation of most California wetlands (Figures 2a and b):

Plac ' en tun t qlJi l l

During the period 1916 1966, wetland areasexpe- rienced considerable decline as a result of" fill placement. The northern areas were /]lied to accommodate westward industrial growth in Na- tional City. In the mid-1960s, a large area of marsh was covered with dredge spoils (referred to as the D-Street Fill).

76

San Diego Bay

;

Street Marsh t / " - ~ ' ~ 7 - {

~ l i _ , , l e r Pond j

.,- ..... ___ , , , ) 0 "=~ . . . . ,>

S w ~ R~ [~k~mb

Jt ~" /

i <",i

D Street Fill <O0,a rea, " , . .

; / ~"r )

i. arisma',, j _de Nacion 9 ~ ~ "

.Connector Marsh

L E G E N D

x Profile Point 9 Tide Monitoring Station 9 Benchmark

Cross-Section Contours at &Feet Intervals

- - 0 - - - 5 . . . . 10

,5 "

_ . ~'~ . . . . . . . . . . "Y~5, Paradise M ars h - ~

In ters la te 5

Fig. 2a. Sweetwaier Marsh National Wildlife Refugc (SMNWR): 1995 conditions.

,4/teratio#l of llw ci#'culalion .s'r.s'tem

The PC wetland formerly fl-onted directly oll San Diego Bay, with extensive bare mudflat and low- marsh area, and an overall east-west marshplain slope. The filling of western marsh areas required the re-routing of tidal circulation in tile remaining marsh through a narrow channel to the Sweetwater River more than 5 km fiom tile Bay. In 1990, tile Sweetwater River was diverted into a major flood control bypass channel constructed between the CM and PC wetlands. This new tidal connection to tile Bay has once again dramatically altered internal circulation.

Diking/vutzs'idence

During the early part of tile century, two access roads to Gunpowder Point were constructed that isolated a portion of the wetland (Vener Pond) from regular tidal exchange (Fig. 2b). Subsequent drying resulted in the subsidence or'the marshplain by about 0.7m. Extremely limited tidal exchange continued to occur via a small cuh,'ert throu~,h the

south levee. This produced a combination of unvegetated mudflats and ponds that were flequent- ly used by shorebirds, although unnatural in tidal wetlands. However, tile north levee gradually subsided until it reached a low point within the level of the tidal range (probably in tile late 1970s and early 80s), which has gradually re-initiated signif- icant tidal exchange between Vener Pond and Sweetwater Marsh.

,41teratioll (#/the local arm regional./lm,ia/ regime

A large dam was constructed oll the Sweetwater River in tile upper watershed in 1888. This has eliminated much of" the natural freshwater inflow and sediment discharge to the study area during most years. Oil a regional basis, tile other major streams that historically conveyed freshwater and sedinlent to the Bay have been dammed (Otay River) or diverted away fl'om tile Bay (San Diego River). As a restilt, the sediment inflow to the Bay and suspended sediment in tidal flows are extremely low. The fine-grained sediments that are characteristic of many tidal wetlands are mostly

77

Fig. 2h. An ADAR image of the SMNWR.

78

/.--

/ ; L -- - 9 '; ~/-....~ XS 6

-'--.2

X S ? / "-,.<. 9

/.ig. 3. S u r \ c \ and p i e z o n l e l e r [rallSecls al. the M a r i s m a de N a c i o n and S \ \ e e l w a l e r l l larshes,

absent in tile Bay, a l-'actor that has a l l l a j o r impact on both restoration activities and the long-term stability of tile existing wetlands to accommodate sea level rise or other alterations.

('dtt.l.'~'/l'O])/ll'C cVUI1L';

In 1916, tile Sweetwater River experienced a mas- sive flood as a result of two major rainstorms and the partial thilure of the dam. The results on the doxvnstream marsh were still clearly evident in the first available aerial photo (1929) of" the site. The flood conveyed an enormous sediment load and apparently deposited much of" this over eastern portions of the marsh. While western portions of tile marsh appear to have been undisturbed, tile eastern portion of the wetland has maintained an area of mostly unvegetated salt panne (visible in Figures 2b and 7). Tile elevation of the panne and this portion of tile wetland ranges from +1.3 to +2-m NGVD (National Geodetic Vertical Datum" about 0.3 to 1.0m above the equilibriunl elevation

for marshplains formed by tile deposition of tidally transported sediment), removing it flom tile zone of regular tidal influence (Haltiner 1993).

Tidal wet~and restoration

As mitigation for wetland impacts from the flood control channel and expansion of tile lmerstate Freeway immediately east of tile site. Caltrans constructed tile Connector Marsh (CM) in 1984 and the Marisma de Nacion (MN) in 1990.

The CM is a 12-ha tidal wetland, constructed by excavating sandy dredge spoils. The configuration includes two major north-south channels (to deter access) and a series of islands graded to support a combination of low-, mid- and high-nlarsh. Tile CM was bisected in 1990 by the new flood control channel, which is a wide (65m) and deep (-3m NGVD) trapezoidal channel extending about 3.4 kilometers upstream. MN, all 8-ha tidal marsh also excavated flom sandy dredge spoils, was designed to provide a specific combination of low-,

mid- and high-marsh vegetation. Tile wetland design inclucted a single large meandering slough channel and a relatively Iow-marshplain designed to encourage the establishment of bload zones of cordgrass (Figure 3).

The two restoration sites are intended to provide habitat t\~r two endangered birds (the light-fi-~oted clapper rail and the CalitSrnia least tern) and one endangered plant (the salt marsh bird's-beak).

The SMNWR complex now consists of six interconnected tidal wetlands, each with distinct characteristics that reflect some combination of natural morphology, human alteration and restoration projects.

Physical processes

Tichll Civcu/aUon

Tile mouth of the SMNWR experiences a semi- diurnal tidal regime with a mean tidal range of 1.7m and a spring tidal range of 2.4m, which is comparable to that of the Pacific Ocean, Prior to the construction of tile new flood control channel, all tidal exchan<,ze occurred throu<,h tile Sweetwa- ter channel, with a relatively sinlple flood-ebb pattern in all six of tile wetlands. Tile new channel bisecting tile CM introduced a second point of tidal exchange, and altered the internal tidal flow directions in some portions of the marsh system. Tile revised circulation results in inore complex exchange pattern, with ebb and flood occurring through both channels, altered velocity patterns, and tile f\mnation of null zones, points of zero velocity where flows from tile two Bay connec- tions meet. Tidal data collected during an inten- sive moriitoring period in 1989 and again in 1995 indicate that thll tidal exchange exists throughout the entire channel system, with little damping of the daily high tides. Tile only exception occurs in Verier Pond, where the ebb tide is damped by the surrounding levees. Based on this, the potential tidal inundation-duration-frequency relationship is similar in tile major channels throughout the wetland. However, the lack of a secondary system of smaller slough channels may limit the circulation into tile mid- and high-marshplain m MN. While the morphometric characteristics of these multi- order slough channel systems have been described

79

(Pestrong 1965, (_'oats el at. 1995), their actual role in establishing the root-zone hydrologic and groundwater regime has not been quantified.

The velocity of tidal flows reflects both tile ori- ginal restoration design and the flood control channel. In tile two channels of tile CM, flo x~ I velocity has been higher in the west channel, with indi- cations of lower flow rates, sediment deposition, and bar formation in the eastern channel. A null point was identified at the junction of tile CM and tile old Sweetxvater River channel. Waterflow in MN is toni]ned to tile sinuous rnain slough channel during tile portions of the tidal cycle below elevation of +0.3m. However, as a result of the low elevation of the constructed marshplain, tidal exchan,,e occurs over tile entire nlarsh width when tile \~ater surface elevation exceeds this elevation. As a result, tidal flow velocities are very low, and the marsh appears to fill up and drain gradually, in contrast to other wetland areas where the f'loxx is primarily confined to the channel system.

Fre.vhlvater iJ!lhJ~v

While tile importance of freshwater inflow to tile establishment of wetland vegetation is recognized (Zedler 1983), its exact role in this and other Pacific Coast wetlands has not been widely stud- ted. The fieshwater requhed to lower soil salinity to permit tile establishment of many wetland plants must be supplied by either freshwater inflow or direct rainfi~ll. Tile relative contribution of either of these sources is likely to \ary along an elevation gradient. In response to concerns that tile new flood control cllanriol would permit frosl'Jwator to be IllOlC efficiently conveyed to San Diego Bay (and bypass tile saltn-iarsh), a timber bulkhead was built extending into tile channel at a 45-degree angle to divert high flows into tile sotith CM. Althougli there l-iax'e been a number of high flow events, tile effectixeness of tile strtlCttlrc (and exert the actual need tbr it) has not been evaIuated.

(TH'OIIIIs

Little direct consideration has been given to groundwater conditions in intertidal restoration work. While it is widely recognized that the shal-

80

80.0

70.0

60.0

50.0

.~-'~ 40.0 30.0 I - ~ -

Seawater 20.0 - -

10.0

0.0 k ~ L

0

, /

MN~,

" ~ i \ [ , , I i I I I I

0.5 1 1.5 2 M a r s h E l e v a t i o n ( m e t e r s N G V D )

2.5 3

Fig. 4. Salinity Data at two restored sites and one reference site.

low groundwater zone creates the actual hydrologic regime experienced by the plant roots, it has generally been accepted that providing adequate tidal circulation would create the proper groundwater and soil moisture conditions for wetland plants.

While this may be sufficient at many sites, particularly for low- and nlid-marsh elevation, it was not a con'ect assumption in the SMNWR. The particular combination of climate, substrate and topography produced elevated soil salinities in the high-marsh. To clarify this phenomenon, groundwater conditions were measured in three transects, two in constructed wetlands (MN and CM) and one in an undisturbed and well-vegetated area of the Sweetwater Marsh. The restllts of monthly monitoring (Figure 4) indicated that while salinities in the low-marsh zone of the constructed wetlands were comparable to the reference wetland (and seawater), a condition of groundwater hypersalinity exists under the high marshplain in both MN and the CM island, (between elevations of 1.0 and 1.7m NGVD). The infrequent tidal inundation, high evaporation rates, and coarse substrate all

contribute to the high salinity, but these are regional parameters affecting both the constructed and the reference sites. If one considers that the reference transect (with similar conditions) was vegetated throughout the high marsh range, then is becomes clear that additional factors are likely involved. The i-briner dredge spoils had a high initial salinity, which was not moderated as a result of infi'equent tidal inttndation in the high marsh zone. Also, the lack of an initial plant covet may contribute to higher soil evaporation rates, as a result of direct heating of the soil surface (Bert- ness el a/. 1992). Although a large salt panne has existed on the Sweetwater Marsh during the past 80 years, we attribute this to topographic conditions: when the sediment was deposited during the I916 flood, it raised the new marshplain out of the zone of frequent tidal influence. While most of the raised area now supports salt marsh vegetation, the central portion of the marshplain developed shallow ponding conditions, without drainage channels. Ongoing evaporation concentrates salt at the soil surface.

Even more surprising was the occurrence of

A. Static

'e_=/

-- -=UlMMUUMIIIIMI T I M E

C Progressive Change ,.,. a : I dynamic.,.. ,

gradual

lllmllllllllllllllllllllllllllllll T I M E

Fig. 5. Alternative ecosystem dynamics.

very low salinity/brackish groundwater at the tipper edge of the marshplain around the peritneter of MN. This was indicated by the establishment of willow species. We have occasionally observed this phenomena at isolated groundwater seeps or springs in saltmarsh settings. Since there does not appear to be source of regional fi'esh groundwater at this location, it is likely that rainfall infiltrates the surrounding fill and emerges near the base of the excavated slope around the perimeter of MN.

The above occurrences suggest that a variety of conditions may control soil salinity. At any one location, relatively minor differences in topography, soil, or local setting may initiate a process leading to higher or lower salmity.

Geomorph ic evolution

In tidal wetlands, the site topography creates tile particular hydrologic regime that detemmaes vegetation type and establishment. In recognition of this, the MN and CM sites were graded to a configuration designed to support the desired percen- tages of low-, mid-, and high-marsh to achieve permit conditions. However, unlike upland sites, where graded conditions may persist indefinitely, tidal wetlands morphologically evolve towards a dynamic equilibrium state in response to the particular combination of hyd,-ologic and sediment regimes (Myrick and Leopold 1963, Pestrong 1965).

B. Dynamic Stability r l , . m,.' O t u

r e,,-

~ o . . .

T I M E

D. Catastrophic Change

lilli,iillilliliiililiiliiliiii r ee" . = , . . (

T I M E

81

Depositional processes tend to create tile marshplain at an elevation of approximately MHHW, and a tidal drainage network with channel density, order, and cross-sectional characteristics depen- dent primarily oil tile area of wetland and the local tidal characteristics (Haltmer and Williams 1987, Coats el al. 1995). When either tile local or regional external physical processes (such as tidal characteristics or sediment supply) or tile internal conditions (such as marshplain elevation or channel characteristics) are altered, the wetland will re- spond by evolving towards a new equilibrium, via the processes of erosion or deposition. Tile nature of the response and associated time frame can vary dramatically between sites (Figure 5).

In the Sweetwater Marsh complex, numerous external and internal changes have OCCUlTed during the past century and all of the wetlands are under- going both geomorphic and biological evolution. These chan,,es are occurrine, at very different temporal and spatial scales. For example, topographic restllts fl'om the 1916 flood or direct filling of tile marshplain are immediate. However, subsequent channel response by deposition will be slow, because of the low concentrations of available suspended sediments. Conversely, the rate of change due to erosional processes will vary de- pending on the shear stress of tile scour and tile resistance of tile substrate. In general, erosional processes will be directly related to the magnitude of tile tidal prism exchanged. The following types

82

1.0

,m ;:> 0.5 0 Z g

0.0 o

m

-0.5

-1.0

S O U T H ; : ', : N O R T t t . . . . . . . . . : . . . . . . . . . @, P, e_cl!c_t_e_d_ie!u_i_l!br_iu - _m_ i . . . . . . .

:: .... ..7"--

. . . . . . . . . . . i ........ :itt i - ; ,7

16 32 48 64 80 Distance (m)

l"ig. 6. Existing and predicted Iimlre cross-section o f Maris- ma de Nacion.

of evolution are occurring at the various altered sites in the SMNWR.

Diked. sub,~'ided and hreached wellands. The diked, subsided wetland, Vener Pond, is rapidly evolving in response to the increase in tidal circulation that Ires occurred as a result of the levee subsidence and breach. The lower extent of its tidal range has decreased from +0.7m (1989) to -0.05m (1995). The internal tidal channels are incising, and the formerly bare mudflats are becoming vegetated as tidal drainage reduces the duration of soil saturation. The marshplaill (which had subsided about 0.7m) will gradually be raised by deposition of suspended sediment to a level of approximately MHHW, although it may take up to 50 years because of the limited sediment supply. The primary slough channel providing tidal exchange with Sweetwater Marsh will likely experience a complex response: the channel cross- section and depth, which were initially small are rapidly increasing at present, in response to the large tidal prism (which is significantly greater than that of a natural wetland because of the marshplain subsidence). As deposition gradually raises the lnarshplain and reduces the tidal prism, the channel cross-section will contract.

.4/teratioH ~)/ the tidal circulatioH sv,s'tem. Tidal prism exchange, flow velocity and directions in the main slough channels of the SMNWR have

been altered by prior filling, the construction of MN, the increased tidal exchange in Verier Pond, and by the construction of the new flood control channel. This latter project created a second source of tidal exchange with the Bay, and results in a more complex tidal circulation pattens: previously, a simple ebb-flood pattern occurred in the system, while tidal water now enters both the Sweetwater Marsh and flood control channel with a "null point" of no flow (near the Railroad Bridge Tide Station, Figure 8). As a result, sediment deposition will gradually occur at this point, and the present large channel will be filled. Although the rate of change is slow, the presence of a loose floc of deposited sediment is already evident in this area. The dimensions of all adjacent slough channels will evolve in response to these changes (Haltiner 1990).

Fluvial d~7)osilion. Tile PC marsh formerly drained directly west to tile Bay, and the marshplain reflected these conditions with an east-west slope. The western areas of the site were formerly directly next to the bay and included large areas of lower elevation open mudflat and low-marsh vegetation. As a result of the altered tidal circulation, this marsh is now well inland from the tidal source, at a relative position more likely to support mid- or higher-level marsh. Sediment deposition is gradually raising the lower portions of the marshplain. As a consequence, the formerly bare mudflats now support extensive areas of cordgrass. Consistent with our experience on other sites where the marshplain is raised by the deposition of fine-grained sediment, the subsequent vegetation growth is excellent. However, this is also a transient condition reflecting the present mo~l~hol- ogy of an evolving system. Continued sediment deposition will gradually raise the marshplain to an elevation more suitable for mid- to high-marsh species (primarily pickleweed). The cordgrass may persist only along channels.

In the western portion of the Sweetwater Marsh, the salt panne area, raised out of the tidal range by fluvial deposition, has remained unvegetated for 75 years. However, tidal slough channels have recently extended into the salt panne via a process of headcutting fl-om a recently excavated mitigation channel. The increased tidal circulation is

3

t"i'..,,. 7. Mariscna de Nacion. (olmeclor . and Se, cclwalcr mal-shcs.

reducing the soil hypersalinity and increasing soil moisture, thereby allowing subslanlial vegetation encroachment.

Re.s'cored u'et/amL~'. The evolution of MN and CM are of particular importance to the mitigation requirements of the restoration project, and we point

out tl~rce limitations of shoil-term compliance standards.

, Tile MN marsht~lain was graded to low elevations to promote cordgrass establishment and to enhance clapper rail habitat, However, this is not a natural or stable elevation f'or Ibis geo-

84

= I U)

._ .~ % ~%

o = "- ~- " ~ ~ t

~II " " ' I=i ~'

I }!

' / e J

4._ / / ' , . ,: . . . . . . . . . ".,._J~_,,.-" I~. ,, ......... ,;-' ~ ~

leuueqo IO.qUOO pool..-I

j ~ , ~ oO

~ I -~ ' 7 ~ | , !-~::... - c~ 9 : .~ . . . . . ,_~ II ) t-.,..,' -" -t~ ;1s ,:.:~-I\ ' j ," --"--:: , , ' 4r-I ', 8,~ ;J,~ ' ~ ' : (~ r" \~'{I ~ ~ . ~ . - : ) ; J ~ ~ :".-' ,,

/ ~ I ', ) .

A---" 7 ~ ' " ' / i

i /

i

I -= s

,x , / ,I"

z

,-j

9

G

r j

9 E c

r~

85

Tahle 1. Results o f assess ing constructed areas to determine if they meet the home range requirements established for the light-footed clapper rail (from PERL 1995). CM Connector Marsh, bdN - Marisma de Nacionl.

Potential Meets cordgrass Potential Potential home Potential home home range: nest ing patch range has 15% range has 15% range

standards? high-marslf? Io~-marsh? Meets all standards?

1 (Clll) yes n o yes no 2 (cnl) 11o yes yes no 3 (cm) no yes yes no 4 (cnl) no yes no ilo 5 (ran) yes yes yes yes 6 (ran) yes no yes no 7 (nln) no yes yes no

.

morphic feature. Over time, we expect deposition to raise the marshplain 0.3 to 0.5m (Figure 6). Thus, even if the cordgrass performance standard were achieved on a short-term basis, it would not likely persist as a stable landscape element. While we expect the marshplain of MN to be depositional in character, portions of it are cur- rently eroding. As a restllt o f the combination of the low-marshplain and an overly sinuous main channel, a series of broad meander cutoffs are developing across the "'peninsulas" created by the channel shape. These areas of surface erosion are 30-60m wide, and were created by the over-marsh flow on the ebb of high tides. Although the surface flow velocities are mostly very low (rarely exceeding 0.15 raps), these have been sufficient over a period of years to create an erosional, humnaocky topography, mostly devoid of vegetation (Figure 7). Eventually, these broad cutoff zones will create a new, less-sinuous and smaller primary slough channel, with a system of smaller, higher order channels. As that occurs, we would expect the marshplain surface to become depositional and be raised. It should be noted that we have not observed the development of these cutoffs and the overall erosional conditions of the marshplain surface on other wetland restoration projects; indeed, the approach of creating a marshplain slightly lower (by about 0.16m) than the projected equilibrium elevation is generally recommended, at least in wetlands where there is a high suspended sediment concentration. The overall

.

shape and circulation in MN encourages detrital deposition around the perilneter in the high marsh zone. The ongoing deposition of dead plant material should eventually provide a higher level of organic material in the marsh sediments compared with marsh areas where high velocity channel through-flow discour- ages detrital accumulation (such as the CM). In the CM, the easterly of the two channels is gradually silting in, and it now conveys mini- real flow during tidal exchange (Figure 7). This indicates that geomorphically, this type of configuration will probably not provide a long- term water barrier to feral animal access. The westerly channel, which was excavated straight, is beginning to develop a meandering pattern, eroding the outside of bends in some locations, and depositing sediment elsewhere. The tidal circulation in the CM restilts in generally high velocity flow through the marsh channels, with less opportunity for detrital deposition and organic matter accumulation compared with the shape of MN, which encourages deposition. Some deposition between islands has been noted.

The overall patterns of geomorphic erosion and deposition within the SMNWR system are shown in Figure 8. Even if no additional modifications are made to the marsh system, it is evident that many decades will be required before the system evolves into an overall equilibrium configuration, with numerous topographical and hydrologic alterations. Vegetation patterns will continue to change m response to this topographic evolution.

86

14

12

10

8

C5 6 Z 4

100%

75%

50%

25%

0%

P O O L E D SP. R I C H N E S S

[] Constructed O. ""'\ 9 ...............

/ /.'

I I I I I I I

RELATIVED COMPARISON (Constructed/Natural)

I I I I I I I

~ ' , ~'~ 0 ~, 0",

(.~

12-

2'

300%

200%

100% -

0 % -

M E A N T O T A L D E N S I T Y

T . ~ 1 _ _ 1 l J 1 _ _ _ / _ _ / _ . _

- - T ! 1 " t - - r - - T - -

Fi,k'. 9. Fish species r ichness.

Bio log ica l f u n c t i o n s

Mitigation criteria have been met lbr the Califor- nia least tern (i.e., chap.nels that provide fish as forage) and the salt marsh bird's-beak (i.e., a population that sustained itself for three years). In addition, some of the standards for the light- footed clapper rail have been met. However, because low and high intertidal marsh vegetation is limited in both extent and quality, the constructed marshes /hll short of achieving the criteria tier mitigation compliance {Table 1). From an ecosystem-functioning perspective, even where standards have been met, there are many differences between the constructed and natural wetlands (e.g., fish habitat types, bird's-beak population dynamics).

Cal!/brnia /east tern

The California least tern is a migratory species that nests on the D Street Fill and leeds on small fishes in nearby subtidal areas. Criteria for mitigation

compliance were that channels provide foraging area for the tern, with the goal that constructed channels have 75% of the native fish species rich- hess, and 75% of the density found in natural marsh channels.

Fishes rapidly occupied constructed channel habitats. which lnet or exceeded the established criteria within three years of monitoring (Figure 9). Most species, with the exception of striped mullet (Mugil c~.7)llahcv) and Northern anchovy (Engrau- lis mor&Lv), occurred in both natural and constructed channels. Differences in numbers of species or total fish density in channels were not de- tectable by channel status (constructed versus natural), but there were inte,annual differences (althou,,h no directional trends). Althou~h USFWS biological criteria were easily met, measures of individual species abundance and relative assemblage structure in constructed channels were not equivalent to those in natural wetlands. The proportion of two species differed significantly in

catches fi-om natural and constructed sites: Long- jaw mudsuckers (Gillicl~thvs mirahili,v) were relatively more abundant in natural reference channels (43% of total) than m samples fl'om constructed channels (16%), and the California killifish (Fundulus Imrvit)imHs) comprised more of the catch in constructed marsh channels (36% of total) than in natural channels (14%).

Tile composition of fish assemblages was related to particular physical parameters of channel habitats, rather than their natural vs. constructed status. Mudsuckers predominated in the more narrow, steep-banked channels with high clay sediments, elevated salinities, and low dissolved oxy- gen levels. Killifish dominated samples fl'om broad and shallow channels with low slopes, fiinged by emergent vegetation. Samples in which top smelt (,4tlwrinol)S q[/inis) predominated were generally fi'om broad, deep channels (mean depth of 1.2 m). We recommend that restoration criteria be tnore specific to individual habitats and that fttture studies attempt to contro~ t\~t" hydrology across "'treatments" to better measure the biotic effects that can be solely attributable to habitat alteration.

Channel physical characteristics and hydrologic classes were diverse and not equally represented in the constructed and natural wetlands wherein fish were sampled. Constructed channels tended to be wider, more gradually sloped, and deeper than those in reference wetlands and most could be classified as lower-order stream channels. Addi- tionally, these sites lacked small tidal creeks and all received urban runoff fl-om storm drains. Chan- nel morphometry, stream order, proximity and type of channel vegetation, flow rate, and sediment type are all known to influence tile occurrence and abundance of individual fish species (e.g., Mclvor and Odum 1988, Baltz 1993, Paller 1994). Further, the absence of small creeks is likely to reduce tile potential for fish nursery flmc- tions (Desmond, in preparation). While tile criteria for mitigation have been satisfied, the constructed channels may not be t\mctionally equivalent to those of natural wetlands with regard to individual fish species abundance and assemblage structure. Future projects should sel more detailed criteria for the habitat type to be constructed, pre- ferably including small, low-order streams in the desiml

87

Channel morphology appeared to be influenced both by the channel 's alteration history (natural vs. constructed) and its hydrolo,,y~ . Three of the natural channels chosen as reference sites were small, narrow creeks classified as higher order streams than most of tile created marsh channels. As a re- suit, interpreting the effect of channel creation on the fish assemblage was complicated by hydrologic inequities in each "treatment" (natural vs. constructed).

Salt ma/wh hird'x-heak

Standards for tile endangered plant, salt marsh bird's-beak, were met ill 1995: populations were stable or increasing for three years. However, tile high marsh overall has not yet met the mitigation standards requiring sufficient acreage of high tide refi, ge t'or clapper rails. Several abiotic problems have been noted: Along CM, higher marsh is eroding next to deep and steep-edged channels (bank undercutting): surface soils develop salt crusts: the ground water is hypersaline: soils are very coarse in texture: and sedimentation has occurred in low- marsh areas.

Several concerns about the high marsh habitat have been identified. First, the area of appropriate elevation tbr high marsh is declining. CM was designed to have large channels running along the east and west perimeters. However, the west channel has been deepening (both north and south of tile flood control channel), and the soils along the west edges of the marsh islands have been eroding, resulting in steep cliftA in some places. As the soils erode, high marsh vegetation is being lost. Although tile appropriate topography for high marsh may be developing in other areas as marsh sediments accrete, this gradual process is not likely to keep pace with tile current high rate of erosion of steep slopes.

Second, the high marsh vegetation has relied to establish in several critical areas, even after 12 years. As a result, there is insufficient high marsh to meet home range criteria (PERL 1995). AI least

- O / ) ~o of each home range must support high marsh vegetation to provide cover for light-footed clapper rails, which escape high tides by moving to vegetated areas of the upper marsh. A white sur- tilce crust indicative of high salt content is appar-

88

ent on the bare island tops of CM, and soil salinities can reach three times tile salt content of sea water (PERL 1995). Hypersalinity of soils may limit seed genninatioll and establishment of vegetation (Kuhn 1995). Once established, however, mature halophytes can withstand hypersalinity. For example, the highest soil salinities found during our annual monitoring in September 1995 were along a high marsh transect with very high vege- tative cover (1% open space) (PERL 1995). While this transect lies within the constructed marsh, it maintains vegetation from a remnant marsh established sometime before construction.

h-rigation might help to reduce salinity and aid vegetation establishment, especially if the soil is soaked for several weeks during the spring when plant growth is rapid and plants gain salt-tolerance. However it is not clear how much water is required, as even tile high rainfall of winter 1994-95 produced only a brief decline in soil salinity. At tile end of tile 1995 growing season (September), soils were two to three times more saline than in March 1995 (PERL 1995). We do not yet understand the interaction between salt crusts, hypersaline groundwater, and high-marsh plant growth. Plants may either concentrate salts in tile soil profile, by excluding salt at tile root surface, or dilute salts by taking them tip and depositing them oil the surt'ace. Too little is known about rooting depths, root densities with depth, and salt uptake/exclusion by roots.

Light:fi~oted clal~per rail

Low marsh fails to support tall cordgrass, which clapper rails need for nesting. A persistent problem in the constructed marshes at SMNWR is that tile cordgrass (Sl)artina .fidio.va) is too short. Where clapper rails nest in natural marsh canopies, there are >90 stems per square meter that are taller than 60 cm, of which >30 stems are taller than 90 cm (Zedler 1993). In contrast, the vegetation canopy in CM has consistently been shorter, even eight years after transplantation. To date, there has been no nesting by the endangered light-footed clapper rail in constructed marshes. Short cordgrass has also been linked to scale insect outbreaks in the constructed marshes of San Diego Bay

(Boyer and Zedler 1996). Part of the cause may be the paucity of tile scale insect's major predator (a coccinellid beetle), which appears to need tall cordgrass to escape high tides. The tall cordgrass canopy of the adjacent natu.'al marsh (PC), has many beetles and very few scale insects (Boyer 1994, Boyer and Zedler 1996).

Tile poor cordgrass growth in Sail Diego Bay constructed marshes has been explained by low levels of nitrogen in tile soil (Langis el ell. 1991 ). Nitro- gen concentrations are low in part because decom- position and leaching rates are both high in the site's sandy soil. Cordgrass is nitrogen limited, with both foliar nitrogen concentrations and bio- mass increasing following urea additions (Covin and Zedler 1988, Zedler el al. 1992, Boyer and Zedler in preparation). However, fertilization with nitrogen has not solved the problem. Cordgrass at CM met Zedler's (1993) canopy height criteria after several different experimental fertilization regimes in 1993, but effects were not sustained unless amendments continued year after year (Boyer and Zedler, in preparation). Further, when three large areas (300-400 m 2) were fertilized in 1995, only one met tile canopy height criteria by August 1995 (PERL 1995). Cordgrass in tile other two areas appeared to be in competition with salt wort (Bati.v marifima) and annual pickleweed, (&llicor- ni l Digelorii), which may have limited tile success of the cordgrass.

Soils are not accumulating organic matter or nitrogen oil their own" l\mctional equivalency is not likely soon. Our long-term sampling of sediments in the CM shows that organic matter pools are developing very slowly. Sediment organic matter levels show an average increase o f - 0 . 5 % per year, ranging flom approximately 4 5% during our early studies (when tile marsh was 3 4 years old) to -8% in 1994 (at age 10 years; data for north islands). It is not clear flom the scatter of points whether the high vahie in 1994 is an anom- aly or part of a continuing upward trend. Relative to PC, sediment organic matter in the constructed marsh appears to have increased, but tile mean is still less than 75% of that in the natural marsh. Despite improvement in the sediment organic matter pool, tile constructed marsh soil has not accu- mulated nitrogen, and total Kjeldahl nitrogen

(TKN) levels have remained very close to 1 mg N/g since 1988. Sediment TKN concentrations in tile CM have been about 50% of the levels in PC over the entire study period.

The coarse soils at CM likely limit tile accumulation of both organic matter and nitrogen in the soil, and it is only with the current accretion of finer surface sediments that the site is likely to show substantial improvement. However, as noted elsewhere, accretion of sediments carl shift the plant community from dominance by cordgrass to pickleweed.

Fertilizers can increase plant heights, but results are brieE Plants and their rhizospheres fail to accu- mulate and retain nitrogen as N-rich organic matter. In our fertilization experiment in 1993 and 1994, soil nitrogen did not increase with amendments and we hypothesized that below ground tissue N storage after- aboveground senescence was also poor, with little N available to new shoots the following spring (Boyer and Zedler, in preparation). We evahiated below ground tissue N in an- other experiment in 1995 that used the same fertilization regime. Contrary to our hypothesis, the below ground crop of N in the constructed marsh was equivalent to the reference natural marsh after one year of alnendments (PERL, unpublished data). The results of the two experiments suggest that amendments improve N storage in below ground tissues in tile constructed marsh, but that these stores are not adequate to match plant growth in natural marshes or to sustain growth. There are larger pools of N and organic carbon in the fine-textured soils of the natural marshes than in the constructed marsh, and natural soils appear to supplement cordgrass growth as below ground tissue stores are depleted during the growing season. The sandy soils of the constructed marsh may take many more year's to develop the characteristics needed to support tail cordgrass canopies.

Areas of cordgrass marsh are accreting sediments and shifting to mid-marsh vegetation. The CM is accreting sediment in several locations near the eastern channel and some islands are merging as sediments fill in the channels between them. In one large-scale fertilization site intended as a potential clapper rail nesting patch, sedimentation is causmg the plant community to shift toward mid-marsh vegetation: what was once nearly pure

89

cordgrass is now becoming dominated by salt wort and annual pickleweed. Accretion of just 10-20 cm of sediment can shift the low marsh to a mid- marsh plain. Therefore, it is unreasonable to expect cordgrass to persist in large areas where sediments are accumulating. Similarly, we have seen cordgrass distributions shift laterally, as documented by markers fl'om a 1993 experiment. Flags were placed in a band of pure cordgrass near the edge of the eastern tidal channel: cordgrass now extends several meters beyond the flags. These former channel-edge mudflats are gaining cordgrass as sediments accrete.

Lessons learned

Early establishment of high-marsh vegetation ira constructed marshes appears to be very inaportant, because seeds and seedlings have low tolerance for high salinity that may develop on substrates without plant cover. More research on interactions of vegetation and salt crusts is needed. Dome- shaped islands may f:avor salt crust formation, and fiat or more gently sloping marshplains, like those of natural marshes, are recommended. Fine- textured substrate and organic top soil should be salvaged from any sites that will be damaged by construction activities: it should be stockpiled and reused in the restoration or mitigation site. Be- cause coarse texture limits organic matter and TKN accumulation, it is unrealistic to expect that a site with sandy soils will achieve functional equivalency with natural marshes having soils with high clay content.

The geomorphology and biological responses of both mitigation sites are very dynamic and difficult to predict in detail. In general, we have come to recognize that large areas of homogeneous low-marsh habitat can be constructed but should not be expected to persist. Marshplains tend to reach an equilibrium with the mid-marsh elevation, which supports dominance by pickleweed, rather than cordgrass. To provide large areas of cordgrass for the light-footed clapper rail will require construction of tidal creek networks that will retain low-elevation edges through tidal action. Construction of dense tidal creek networks, following tile topographic complexity of natural

9O

marshes, should have additional benefits for fish reproduction and early growth.

C o n c l u s i o n s

The following observations can be made about the development of the mitigation proiects:

1. Although coarse-grained dredge spoils can support wetland vegetation, establishment may be more difficult and uncertain, and may require a much loni, er,., time flame to develop appropriate substrate chemical and physical conditions.

2. In an environment in which the substrate is low in nutrients {such as dredge spoils), a wetland design that encourages detrital accumulation (i.e., "dead end" slou<,h channels), will eventually develop a more organic soil than a "f low through" wetland such as the CM.

3. The design of the slough channel system should be based on local reference sites of comparable wetland area, hydrology and topography.

4. Multiple channels (to create islands and reduce access) will not remain stable. One chalmel will become established and others will fill in.

5. The establishment of high marsh vegetation is particularly difficult given a combination of" poor initial soil quality, coarse particle size, and high evapotranspiration rates. Even the provision of an irrigation system has been inef- fective to date. Under these conditions, groundwater and soil salinity may preclude vegetation establishment, unless additional measures are identified.

6. The development of vegetation goals/'success standards should recognize the equilibrium form of the v,,etland, not that initially exca- \'ate&

7. The perl\~rmance of restoration projects is more difficult and prone to failure in a sediment-poor environment such as San Diego Bay, compared with a setting high in suspended sediment such as San Francisco Bay.

8. The potential for catastrophic events, and the likely wetland response must be considered in mitigation site selection and design. In settings

.

where some type of catastrophic events are possible, a higher level of uncertainty in the long-term site functioning must be accepted. When the hydrology and topography of a wetland complex have been altered, the wetland may require many decades to reach a dynamic equilibrium. The performance standards for mitigation projects must recognize this extended time flame, and that short-term (5-year) monitoring will only give an indication of the f'uture form and function of the constructed site.

In conclusion, we observe that restoration proiects must be designed to t\mction in response to both regional and internal geomorphic and hydrologic changes. More importantly, the monitoring, management and evolution of these projects must recognize both the scale and time fl'ame of this evohltion. Perhaps rather than expecting newly restored marshes to quickly replicate the functions and values of natural sites (which have evolved over thousands of years), it is more important to assess whether or not we have created the appropriate "'template" to allow the restored site to develop the desired characteristics over an appropriate time flame. The duration of this time fl'ame may vary considerably between sites.

R e f e r e n c e s

[3altz, D.M., Rakocinski, C. and Flceger..I.W. 1993. Micro- habilat use by marsh-edge fishcs in a Louisiana estuary. Enx. Biol. Fish., 3 6 : l 0 t ) 126.

[~elRlleSs, M.D., (]otlgh, L. and ShumWJ.ly. s . \g. ltJt)2. Salt lolcrances and the distribution of fugitive salt marsh plallts. Ecol., 73:1842 1851.

Boyer, K.E. 1904. Scalc insect damage in constructed salt marshes: Niti-ogen and other factors. M.S. Thesis, San Diego Slate Unixersitv. San Diego, ('alil\lrnia.

la;oycr, K.E. and Zcdlcr, ,lB. 1996. Damage to cordgrass by scale insecls in a constructed salt marsh: effects uP nitrogen additions. Estuaries. 19: I 12.

('oats, R.. Williams, P.B., Cuffe, C.K.. Zedler, ,I.B., Reed, 1)., Wall-y. S.M. and Nollcr, J.S. 1995. Design Guidelines lbr Tidal Channels in ('oastal Wetlands. Report Prepared for the U.S. Arnly ('Ol]~s of Erlginecl-S, \,Vaterways Experiment Station, 45 pp.

( 'o \ in , J, D,, and Zedier..I,B 1988. Nitrogen effects on ,S'parli- #It* . l idi,xa and ,S'~dicol'nia virginica hi the salt nlarsh at "l'ijuana Estuary. ('alilBrriia. Wetlands, 8:51 65.

I lair\her. J.P. 199(). SXVCCtX~,{ItO1 Mcu'sh: MoFphology and Tidal ( i rct i lat ion. Report Prepared for ( 'al l lans l)istrict I I, U.S. Army ('oi'ps of Ellgillt.'el'S. Still Diego, ('tllilclrnia. 130 pp.

lialliner..1.P. 1993. l~Xll-cnlc e\elllS ;iild coastal \\eilands. Pro- cccdings IC;93 AS(' I-hl tc i Inat ional Syinposiunl Oil I{ngin- CCl-ing I f.vdrologx, San Flallcisco. ('ali lOrll ia, Jul\ 25 30

Ilal i iner. ,I.P. and Will iams, PB. 1987. II\ 'draulic design in sail marsh iesioi-alioll. Proceedings Natioilcil S vnlposium el Wetland I lychologv, .&ssociation e l Stcile \Veiland Man- ageis, ('hicclgo, IL, Sept. If~ 18.

I laFvey, T. ;.\lid Josseiyn. M. IgSf~. Wetlands restoralion aild mitigation policies: Comnleilt clnd reply, l{n\ ir. Mgmt, I lk 567 569.

Kuhn, N. 1995. Tile el'l'ccls el 'sal ini ty and soil saturation oil plants in the high inlertidal marsh. NI.S. Thesis, San Diego Stale Uni \ers i i \ . San Diego, (a l i lornia.

Langis, R., Zalejko, M., ~.lnd Zcdler . . I .B. ICB)l. Nilrogen as%seSSlllenl$ ii1 ct COllSlltlcled and a ll~lltlial sah ill~irsh of ,<>till Diego Bay. Cali lornia. l-col. Appl., I: 40 51.

bdclvor. C.(',, and Odum. \\.1{. 108<"4. Food, iuedation risk. and microhabitat selection in ci lllarsh l]sh aSSelllblage. l - c o l . , (~<): 1341 1351.

Myrick, R.M. and Leopold, L.B. I9(G. I lvdraulic (ieOllletl-V of a Smell1 Tidal Estuary. Llnitcd 5relies Geological Stir\c\ Profcssional Paper. 42" B,

Pacil]c I~stuarinc Research Laboralory (PI{RL). 1995. S\',ccl- \~.aler Marsh National Wildlil 'c Refuge: l{cosvslelll Assess- il lelll for Mi l igat ion Compliance. Final Rcporl for It)t)5. (a l i lo r l l ia DcparilllClll o1 TrallSpOl-lalion. ~all Diego. Cali- Ib Fn ia.

Pallor. M.II . 1704. Relationships bot\~ecn llsh asseinMagc Sll-tlClUre aild sleanl order in ,<,otllh ('Cll-oliila coastal plain slrecinls. Trans. i\nl. Fish See.. 123:150 I(~1.

PesilOllg, R. 1765. Tile de\elopnlcnl of dlaiilClgc pallerns on t i&l l illarshes. ,<qlaillol-d LTniversit\, Publiccitions Geol. Sci. \"el. X. No. 2. 87 pp.

91

Race, M.S. 1985. Critique c,l' present wetlands mitigatiOtl policies in the Unitcd States based ,.m at/ analvsis ,.;I past restoration pr,.+j,._'cts in San Francisco Ba\. l!n,.ir. Mgmt., 9: 71 82.

RaCe, M.S. and Fonseca. M.S. IL)95. Fixing compensatory tnitigation: \Vhat wi l l it take'. > Ecol. :\l+pl. 6 :94 lOl.

,VJliahlcr, i \ .N. 19(-,4. Quai l l i lat i \e geonlorphoh+g> o1 drainage basins and channel noi\\orks. In. ('ht+\\. V.T. (cd), l land- book o1" Applied II_vdrology. pp 43(+ 476, Mc(ha\~-I t i l l . Nc~ York.

\Viii\ares, P.B. and Florsheim. J. 1994. Designing the Sono- iYi;.i Bavlallds Plt!iect. (o:.isl aild ()ceai1, Fall.

\Vil l iams P., Sllellherd, A., arid l:abor, P. 19.<'48. k'hmitoring a tidal restOl-tlliOll site in ,Still Fi-cincisco I}av: The Muzzi Marsh. in: Proceedings of tile Naliorlal S_vnlposium orl \ \ct lands "SS: Llrban Wetlands and Riparicul IIcibilai. Association of SLite \Vellands Mcinagers, lgyrnc. No\', "fork.

Zcdler, . IB. 1c)83. t:reshwaicr ilnpacls in normally hypersalinc nl~ll'Shcs. I!sttlaNcs, (~: 346 355.

Zcdler. J.B. 1993. (anop.x archilecltu-c o[" natural and plamed cordgrass nlalshos: Sclcclillg habitat e\alt ial ioi l criteria. Ecol. Appl., 3 :123 17<<4.

Zedlor, J.B., Nordb,,, S., alld KtlS. 17.1{. IC;C)2. The l_:cology of Tijuana I{siuarv: ,,\ Naliona] and Esiuarinc Research Re- :4OI'VC. NO,'\,'\ Ol'l'ice of ('tlasl;ll Rcsotlrce Mallagenlenl. SallCtUal'ies and Rcser\cs l ) i \ is ion, \Vashington. D.('.

Zedler. ,I.B., Principal iktilhor. 1996. Tidal ~xctlaild icstora- lion: A scieniillc perspective and sotilherll ('alil\x-nia l\+ctis. (cilil\+t-nia Sea (h-tull (o l lege System. Lhliversitv e l Cali- lornia, La ,Iolla. ( 'a l i lornia P, el+Oi-I No. T 038.

Manuscript recci,,od: IN July ]99(~ Manuscript accepted: 16 October 1'-)9(~ Corrcsporaling editor: R.E. Turner

Documents

Influence of physical processes on the design, functioning and