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Common Wetland Rehabilitation Techniques

Backfill Ditches. Ditches are usually dredged through wetlands to promote irrigation and move water. The dredged material is often piled along the edge of the ditch in piles, known as spoil banks. These can block sheetflow (The movement of water across a surface in a sheetlike mass instead of within channels or streambeds). Backfilling can help restore the natural topography and hydrology, which can lead to at least partial rehabilitation of wetlands affected by ditch construction. However, completely removing the material from spoil banks can be difficult. In older ditches, especially those dredged through organic substrates, spoil banks can oxidize, which reduces the volume of backfill. In many cases original elevations cannot be achieved.

Construct Berms. Constructing small dikes or berms can protect adjacent property from the natural flooding of a restored wetland.

Using fire to suppress weeds and encourage growth of native seeds, Gotter Prairie, Hillsboro

Using fire to suppress weeds
and encourage growth of native
seeds, Gotter Prairie, Hillsboro

Control Weeds. Thousands of plant species have been transported beyond their natural ranges, both intentionally and unintentionally. Many introduced species spread prolifically in environments where predation and competition are limited, pushing out the native flora.

Undesired plants or weeds can be pulled manually or mechanically or eradicated with herbicides, grazers or pathogens. They can also wither through manipulation of the hydrology, or by a combination of methods. Weed control can allow reestablishment of native plant communities. However, once introduced plant populations are well established, removal is a labor intensive, ongoing task.

A combination of biological, manual and mechanical controls, herbicide application and hydrologic manipulations may be required to eradicate the invasive species. Control using herbicides is not always appropriate. Chemical herbicides may damage the native species as well as hinder the establishment of native vegetation. Hydrological manipulation is not always possible.

Black plastic used to solarize and kill reed canarygrass

Black plastic used to solarize
and kill reed canary grass

Using herbicide to control invasive plants at West Eugene Wetland

Using herbicide to control invasive
plants at West Eugene Wetland

Excavation. Excavation can restore natural topography and elevations in order to intercept groundwater, to reach an intertidal level or to establish wetland hydrology. In some cases, sediment previously deposited in a wetland can be removed to rehabilitate the wetland. When landowners strive to create artificial wetlands, they can displace other habitat (i.e., upland habitat). Excavation and removal of excavated material can be expensive. In some circumstances, it is difficult to predict appropriate excavation depths. Excavation to subsoil leaves poor substrate for plant growth.

Water Control Structure at Gotter Prairie, Hillsboro

Water Control Structure at Gotter Prairie, Hillsboro

Install Water Control Structures. Restoring natural hydrology is important on wetland projects. A landowner may wish to artificially alter water levels with a water control structure where bullfrogs or other invasive species need to be controlled, or when the natural hydrology cannot be reestablished. Risers or stoplog structures made of plastic or metal help manage the normal flow of water. If excess water is expected during flood events, a wide troughlike opening in the side of the dike called an emergency spillway should be designed into wetland projects. These spillways allow water to pass through without damaging the retention structures during a highwater event.

Because water management is critical, it is important to determine how much if any water should be controlled in your wetland. Water control structures generally require a lot of maintenance and monitoring, and may create fish passage barriers and entrapment.

Reconnect Floodplains and Restoring Backwaters, Channels and Bends. Restoring a stream to its natural channel, and reconnecting channel and floodplain, can reduce sediment load and flooding downstream; raise the water table; lower water temperature; and restore fish and wildlife habitat.

Historic oxbows, now disconnected from the mainstem of the Little Deschutes River, La Pine

Historic oxbows, now
disconnected from the
main stem of the Little
Deschutes River, La Pine

Productive backwaters, side channels and meanders can serve as a refuge and nursery for young fish and other aquatic life and improve adjacent wetlands. To reestablish these bends in the river, a landowner can modify or remove barriers, such as flood levees, roads, fences, farm tracks and earth banks. Backwaters and side channels that receive water in high flow events help stabilize banks by reducing erosion. They also recharge groundwater, create habitat for a variety of wildlife species and provide refuge for fish during flooding. A straightened stream can be reconnected to parts of its former meandering channel by removing dikes or levees that keep it in check. The historic channel can be identified from old photos or by the presence of a residual line of vegetation. This may require heavy equipment, and assistance from a geomorphologist or hydrologist.

Breaking drain tiles, Willamette Valley

Breaking drain tiles, Willamette Valley

Remove Culverts. Removing or repairing culverts can be an effective way to increase fish habitat. Culverts can block fish passage by constricting flows; collecting debris that plugs passage; and forcing the water to find another path, often one that a fish can not follow. In many coastal areas, roads have been built across tidal creeks, separating tidal wetlands from the estuary. Frequently, tidal creek flow is maintained by the installation of culverts or pipes that pass beneath the road. However, these pipes are often too small to allow full tidal flushing of a creeks wetland, which leads to a reduction in its size and changes the vegetation Further, if a culvert has been installed with the bottom of the culvert above the level of the creek bed, the culvert acts as a weir, holding water on the wetland. This may have caused loss of plants and in some settings, hypersalinity.

Remove Tile. Tile breaking during wetland projects involves removing a section of underground agricultural tile that drains a wetland basin. Drain tile or field tile is usually made of clay or perforated plastic, and buried at a depth of two to six feet. Generally, a backhoe is used to remove or crush a 25 to 50foot section of tile downstream of the basin. The downstream end or outlet pipe is then plugged with a bag of redimix concrete or clean clay fill, and the trench is filled. Sometimes, a portion of unperforated tile or riser is connected to the downstream end of the tile line and brought to the surface in order to control the water level. Water will fill the wetland basin until it reaches the mouth of this riser, where it will then flow back through the tile line into the ditch.

Authored by Esther Lev, Executive Director, The Wetlands Conservancy (2009)