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Wetland restoration

Marshes

The hydrology of isolated marshes supplied by surface runoff is less predictable and often requires a weir or other water control structure to restore the hydroperiod. Weirs are required if water levels are manipulated to control plant communities or if sedimentation is a problem. Restoration of local hydrology to create standing water for marsh development may be accomplished simply by installing a dike to prevent water loss or removing drainage tile installed to carry water away. However, if the marsh was formerly part of a wetland complex, then these alone may not be sufficient to restore former water sources. The other wetlands may have been an important source of water.

Water control structures, such as weirs, are required if water levels must be manipulated or if the wetland is prone to increased sedimentation.

If the water supply is limited to surface runoff, deeper marshes retain water longer than a large shallow marsh where the surface area increases evaporation. The larger the marsh, the larger the runoff catchment area required to maintain the water level of the pond. Any increase in wetland size without addressing additional water will reduce its hydroperiod. The restored marsh cannot be any larger than the original wetland unless water entering the wetland is increased or retained. Retaining water or increasing inflow to the restored marsh may have negative impacts on the water balance of other wetlands in the watershed.

Since it is difficult to balance inflow with loss through permeable soil (percolation loss), it is best to use an impervious substrate, such as clay, that will eliminate bottom percolation. The sides of the marsh depression can be lined from the bottom to a level where water can escape through the sides once a desired depth is ensured. Studies on the rate of water loss to groundwater recharge suggest that recharge is related to the amount of edge, with the most recharge occurring in wetlands with a greater edge to volume ratio. If so, then numerous small ponds may contribute more recharge water than one large wetland.

Growing plants from seed will maintain the original genetic diversity of marsh plants. Many wetland plants depend on vegetative reproduction to rapidly invade wetland habitat. For this reason, the recent development of tissue culture techniques can recreate a whole wetland vegetative community by using multiple seeding lines to retain genetic diversity. Picov's Watergardens, for example, has provided all the propagules for a large lakefront marsh from seeds collected on site from various locations and then multiplied by tissue culture. The community structure of nearby marshes can be used as a model of what species you might plant once water depths and substrates have been determined.

The marsh will develop into a series of communities determined by the variable hydrology (depth of water and how long water remains) of the wetland. Wild life diversity depends on habitats developing at different rates (asynchronous habitat change) and even marginal or aging habitats have a role to play. A mosaic of marsh habitats helps to spread the risk of losing species dependent on a single habitat.

The stress which alters the succession of marsh vegetation must not be so destructive or sudden as to destroy whole communities at a time. If possible, the site should be monitored in advance to determine existing hydroperiod as a guide for site restoration or that of nearby sites. The monitoring should include a description of existing conditions, site history, elevations, hydrology (water sources and destination), hydroperiod, existing wild life communities and the nature of their dependence on the marsh, and adjacent land uses.

Due to an incomplete understanding of wetland restoration it is important to document experimental design and results. Problems are to be expected as we try to factor in the complex interplay of hydrology, soils, hydroperiod, frequency of inundation. A natural ecological instability can be expected in newly restored marshes until a new equilibrium is established.

Wetlands at different successional stages when clustered together create a diversity of plant and animal species; each supports its own assemblage of wild life but also supports species with large home ranges, such as great blue heron. Many small marshes around a larger body of open water provide the variety of habitats necessary to attract a diverse wild life community. Ideally, a large, deep pond adjacent to smaller separate ponds responding to natural precipitation cycles (or with water control structures to control the vegetational community) would provide a diversity of habitats in a small area.

Restoring marshes where they once existed may only require a small degree of damming, but new wetlands may need more extensive dams and dikes to contain water where it did not naturally occur. Water control structures (weirs) are required if water levels must be manipulated or if the marsh is prone to increased sedimentation or low water tables. Weldmesh or stone will deter muskrats from burrowing into dams. As muskrats are part of wetland ecosystems, it is advisable to accept muskrats and design for their impact.

In mature marshes dominated by one or two species of emergents (such as cattails), some manipulation of the wetland hydrology (ie. drawdown or dewatering) may be necessary to return the wetland to an earlier successional stage. Often due to the unpredictable nature of water levels, it is necessary to physically remove cattails. Trenches should be in the shape of gradual sloped basins to maximize surface area for re-vegetation rather than a v-shaped trench or square sided pit that most backhoe or dredge lines create. On-site supervision of slope contours is necessary as backhoe operators can create a smooth transition from deep to shallow water if instructed to do so.

It has been suggested that cattail removal and water level manipulation requires the assistance of someone who is familiar with these methods. As the value of cattail removal can be controversial, we agree that professional biologists should be consulted before altering any marsh.

Ideally, approximately half of the marsh should be returned to open water. If there are nearby marshes in different successional stages, approximately half of the total area should be open water. Some parts will be well vegetated and some areas predominantly open water.

When water is slowly returned to the marsh basin, seeds in the sediments germinate. Initially all species grow equally well but, as water levels are restored, it is the water depth which determines the zones in which plant species will survive in the new marsh. However, marshes should not be considered to be of less value because they have well established monocultures. As stated previously, it is the mosaic of wetlands at different stages of succession which provides a complex of habitats each with its own complement of plant and animal species. Not all wetlands must be returned to a state of higher productivity or diversity. An inventory of local marshes, their species, hydrological function and relationship to nearby wetlands is a prerequisite before restoration is considered.