Wetland Restoration, Creation & Enhancement in the Northwest

by Sarah Spear Cooke, Ph.D. – Cooke Scientific Services, Seattle, WA

Wetlands are elaborate ecosystems which perform many functions in the landscape. A few of the better known functions are stormwater storage, erosion prevention, sediment and pollutant filtration, and the presence of plant communities which provide habitat for wildlife. Even the most diverse team of experts cannot identify all the functional constituents of a wetland because processes within wetlands interact in complex ways. How then do we create, restore, or enhance these systems without thoroughly understanding them? This is a question many managers and ecologists have asked in light of the poor success that has been achieved to date with these types of projects.

Wetland plant communities in the Puget Sound; what have we learned?

Wetland plant communities have been described for Washington by Kunze (1994) and Oregon by Christy (1993). In addition, plant community composition (the different species present in a plant community), plant diversity (the species present and how they are distributed across the wetland), species distribution by wetland size, species richness (the number of plant species), and the specific plant community-associated soils and hydrology, were described in central-western Washington wetlands over a 10 year period (1987-1996) by the Puget Sound Wetlands and Stormwater Management Research Program (PSWSMRP), Azous and Horner, Eds. 1997).

Eleven distinct, typical, wetland plant communities were identified for the Puget Basin. These were found in intermixed groupings throughout individual wetlands (Cooke and Azous 1997). Five of the communities were conifer (Tsuga, Thuja) or deciduous (Alnus, Populus) or conifer/deciduous mixed (Alnus-Thuja)– dominated types. Three communities were dominated by scrub shrub vegetation (Salix, Spiraea, Rhododendron) and three communities were dominated by herbs, sedges, and rushes (Phalaris, Typha).

Some of the important results derived from the 10 years of research were the characterization of the wetlands in the Puget Sound Region. Two hundred and forty-two plant species were identified in the wetlands and adjacent upland buffers in the 26 study sites. Nineteen percent of the species were found in only one wetland and 38 percent were found in 12 percent of the wetlands. There was also no significant connection found between wetland size and the number of species detected in a wetland, a fact which is counter to standard biogeographical theory. In reality, these results indicate that plant species are distributed across the landscape in a random fashion and the loss of even one wetland could have a substantial effect on region species richness. This data is in conflict with standard management practices of allowing small wetlands to be “lost” because it was/is believed that they would be species-poor and therefore of little importance.

Thirty-five percent of the species found in the wetlands in the research sites were shrubs; 25 percent herbs; 13 percent trees; and eight percent grasses, sedges and rushes. Seldom, however, do wetland mitigation designs contain many, if any herbs. Commonly, mitigation projects include only trees and shrubs with an occasional sedge or herb. It was also found that all the exotic species found growing in mitigation, or highly disturbed natural sites, were herbs, shrubs, and rushes. One assumes that failure to include these plants in mitigation designs affords the opportunity for exotic weeds to come in and fill the gaps.

Urbanization processes & their effects on natural wetlands

Most of the plant community characterization done by the Research Program was secondary to defining the impacts of urbanization on wetland ecosystems. Most wetlands in urban areas will inevitably be impacted by urbanization, either directly or indirectly. Direct impacts influence wetlands through physical disturbance such as grading and filling, disking (destroying plants with a weed-whacker), sedimentation (sediments carried in stormwater settling out), or hydrologic changes (flooding or drought resulting from a change or hydroperiod). Indirect impacts can occur through subtle changes in the water regime or water quality. Of all land uses (farming, logging, urban development), urbanization has the greatest ability to alter hydrology (Horner 1997). Changing a wetland’s hydroperiod (frequency, depth, and duration of inundation) can have a drastic effect on the wetland plant composition and nutrient cycling within the wetland (Cooke and Azous 1997).

The factors found to be most responsible for the hydrologic regime in a wetland are the physical shape or type of wetland (open water, flow-through, closed depression) and the watershed surrounding a wetland (the area draining to a single stream or water body) (Horner 1997, Reinelt and Taylor 1997). Surrounding land use (roads, developments and their impervious surfaces, logging, and farming) may have a profound effect on the associated wetland through changes in hydrology and stormwater quality (Reinelt et al. 1997). These watershed characteristics may be further influenced by the inlet (the source of water into a wetland) and outlet (the exit of water from a wetland) configurations of the wetland receiving water from the surrounding basin. Wetlands in the Puget Sound region were found to display four hydroperiods (Reinelt et al. 1997): 1) stable base water level with low stormwater event fluctuations, 2) stable water base level with high stormwater event fluctuations, 3) fluctuating base water level fluctuations with low stormwater even fluctuations, and 4) fluctuating base water levels with high stormwater event fluctuations (Reinelt et al. 1997). Highly urbanized wetlands tend to display the Type 4 hydroperiod, where both base flow and the event flows fluctuate.

Spiraea douglasii

Urban influences on wetland hydrology and water quality can, in turn, affect wetland vegetation. Hydrologic changes resulting from urban influences can have significant impacts on the livelihood of the whole range of wetland flora, from bacteria to the higher plants (Hickock 1980). It is not a coincidence that Pacific Northwest emergent wetlands (those dominated by herbs, grasses, sedges, and rushes) receiving urban runoff are often dominated by reed canarygrass, while non-impacted wetlands contain more diverse vegetation communities (Cooke and Azous 1992). Changes in hydroperiod can cause shifts in species composition, primary productivity (the amount of plant material produced by a plant per unit area), and species richness (Cooke 1991, 1997, US EPA 1995). The most significant discovery of the PSWSMRP research relating to vegetation is that most native plants, and the plant communities in which they are found, are associated with preferred hydroperiod regimes. It is this hydrologic preference that is the dominant factor in the species being present. There was no evidence that water quality or soil showed any connection with species presence in a wetland plant community (Cooke 1991, Cooke and Azous 1997). Although it is known that some species prefer organic or mineral wetland soils, this particular aspect was not examined as a part of the plant studies of the PSWSMRP research.

The Research Program found that most of the native plant species in the Pacific Northwest have a defined hydroperiod requirement for frequency, depth, and duration of inundation throughout the year. The maximum depth during the early growing season (March 1 through May 15). No significant requirements were found associated with plants and the fall, winter, or early spring hydrology (August 16 through February 28).

Plant – hydroperiod requirements

The hydrologic regime of plants and their associated communities determined by the Research Program for all seasons of the year displayed some very interesting trends. Forested communities were the driest communities. Seldom was standing water present in forested areas. Rather, the rooting zone below 6 inches to two feet (12 cm to 62 cm)was usually saturated. Emergent communities were, on average, inundated to 2 inches (5 cm). The scrub shrub communities showed the broadest range of wet to dry conditions across the year.

Tolerance to water level fluctuations (the difference between the maximum water level after a storm event and the pre-storm water levels) was also calculated for the different community types. Scrub shrub wetlands had the broadest tolerance to water level fluctuations across the year. The water level fluctuation of emergent plant communities remains fairly stable across the seasons. Again, this information can be very useful when trying to design a plant community for a wetland mitigation. Very “flashy” wetlands such as retention/detention facilities would probably do best if planted with certain shrub communities (if they dry out by the end of the season) whereas areas with stable water flows might be better as emergent, or if drier, forested wetlands.

Hardhack spiraea was found to tolerate the broadest range of hydrologic conditions throughout the year of any plant evaluated (Cooke and Azous 1997). It appears to tolerate being dry throughout the year to temporarily inundated, to completely inundated throughout the growing season. It is no wonder this was one of the most widely distributed species found in Puget Sound wetlands.

Comparisons made between similar species (of the same genera) can be used to determine which species would be better in a specific mitigation design. For example, two species of sedge, Scirpus microcarpus (small-fruited bulrush), and Scirpus trocinctus (woolly sedge) show distinctly different hydrologic tolerances. Although both grow under similar hydrologic regimes, woolly sedge is usually found in undisturbed wetlands where wetter conditions are observed during the early growing season. More importantly however, small-fruited bulrush is associated with areas of high water level fluctuation throughout the growing season, while woolly sedge has very little apparent tolerance to water level fluctuation. It is easy to determine therefore which species would be more appropriate for a urban-influenced highly fluctuating wetland restoration.

Given the knowledge we have about plant-hydrologic requirements, it is important to know the hydroperiod regime of a wetland area where a mitigation project is going to take place when designing a wetland creation, restoration, or enhancement. Wetlands in urban or urbanizing areas cannot be expected to have the same hydrologic regime as rural or pristine wetlands, without urban influences. Most wetland mitigation projects in the past have had design goals which emphasized the aesthetic rather than matching the design to the existing, or projected hydrologic regime. We now know this is one of the major reasons why mitigation installations have failed. Mitigated wetlands often have a completely different plant community than originally installed.

Water quality and wetland health

Pollutants are introduced into wetlands primarily through stormwater runoff (Stockdale 1991). Decreased water quality in concentrations typical of urban runoff in the Puget Basin, however, does not appear to affect wetland plant life (except for large increases in nutrients). Rather, it appears to affect the wildlife, especially aquatic species such as frogs and insects (Horner et al. 1996).

Invasive plants

One result of changing wetland hydroperiods is the conversion of native-dominated plant communities to exotic-invasive dominated plant communities. Phalaris arundinacea (Reed canarygrass), Rubus discolor [procerus] (Himalayan blackberry), and Juncus effusus (soft rush), to name the worst, are all found with great frequency in wetlands in urban and urbanizing areas of the Puget Sound Region. This is due in large part to their broad hydrologic tolerances to fluctuating hydrology (frequency, depth, and duration of inundation). It was also found that duration of flooding was a key factor in the presence of some invasive species (Houck 1996).


What is needed to improve our wetland restoration, creation and enhancement designs? For one, we need to more closely reflect the natural plant community composition by mimicking “natural” reference sites in species richness, and inclusion of more herbs, sedges, grasses and rushes. We also need to emphasize matching the proposed or existing hydrologic regime to the hydrologic tolerances of the plants proposed to be planted. We have learned that the distribution of individual species or plant communities are related to the hydrologic regime (depth, frequency, and duration of inundation. We have also learned that plant community composition will shift if the hydrologic profile changes significantly.

Author’s note: The papers summarized in this article can be found in a soon-to-be-published monograph: Azous, A, and R. Horner, 1997, “Wetlands and Urbanization: Implications for the Future. Final Report of the Puget Sound Wetlands and Stormwater Management Research Program”, Terene Institute (in Press). Information from the PSWSMRP should be used as a guideline. Although the study lasted for 10 years, the results are preliminary in nature. It is hoped that this research will continue in the future.

Many thanks to Amanda Azous, Rich Horner, Klaus Richter, Lorin Reinelt, Brian Taylor, and all the graduate students who worked on the research program from 1987 through to 1996.


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Cooke, SS,1991, “The effects of urban stormwater on wetland vegetation and soils: a long-term ecosystem monitoring study. In: Puget Sound research ’91: Proceedings, January 4-5, 1991”, Seattle WA, Puget Sound Water Quality Authority, Olympia WA.

Cooke, S and A Azous 1997, “Characterization of Puget Sound Basin palustrine wetland vegetation”, In: Azous, A and Horner, R (Eds.), Wetlands and Urbanization: Implications for the Future. Final Report of the Puget Sound Wetlands and Stormwater Management Research Program, Terene Institute, 1997.

Cooke, S and A Azous 1997, “The hydrologic requirements of common Pacific Northwest wetland plant species” In: Azous, A and Horner, R (Eds.) Wetlands and Urbanization: Implications for the Future. Final Report of the Puget Sound Wetlands and Stormwater Management Research Program, Terene Institute, 1997.

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