Dam and river ecosystem basics module: introduction | hydrology | wetlands | morphology and sediment | water quality | biology | timing of dam repair or removal | references | appendix

Wetlands

Wetlands can be an integral part of a river system, free-flowing or impounded, and represent the transition between terrestrial (dry land) and aquatic environments. Wetlands connected with river systems are called riparian wetlands. There are several definitions, scientific and legal, of wetlands. Definitions of wetlands include descriptions of hydrology, soil, and vegetation. Wisconsin statutes define a wetland as "an area where water is at, near, or above the land surface long enough to be capable of supporting aquatic or hydrophilic [water-loving] vegetation, and which has soils indicative of wet conditions" (Wis. Stats. Section 23.32).

Wetlands have been drained for various uses, especially agriculture. In the past few decades, scientists have realized the ecological value of wetlands for plant and animal species diversity, biological productivity, and for improving water quality. Many initiatives are underway to protect remaining wetlands, restore former wetlands, and even create new ones (Mitsch and Gosselink, 1993).
 

Impounded systems

The construction of a dam and filling of an impoundment floods wetlands adjacent to the free-flowing river, and new wetlands may form adjacent to impoundments as a result of the hydrologic changes caused by the dam. The differences in water chemistry between surface water and groundwater may cause some differences in the types and distributions of plants found in wetlands adjacent to impoundments as compared to the original riparian wetlands.
 

Free-flowing systems

Riparian wetlands are found in lowlands adjacent to river systems. These areas usually occupy the floodplain and are periodically flooded by the river. Riparian wetland ecosystems can have high biological productivity, species diversity, and density. Riparian ecosystems link upland and aquatic ecosystems by collecting sediment, nutrients, and organic matter in runoff from adjacent uplands, and by allowing the movement between the ecosystems of nutrients and energy needed by plants and animals.
 

Wetland restoration

When repairing a dam, the impoundment is drawn down temporarily so that the construction work can take place. Drawdown of the impoundment will lower the water table around the impoundment as the water stored in the streambank is released. As the water table drops, the wetlands adjacent to the impoundment may dry up, depending on how long the impoundment is drawn down and the elevation of the wetlands. Some wetlands will not be lost, but the type of wetland may change because of a change in water level. If the drawdown lasts a long time, wetland plants may be harmed. If this occurs, the wetlands can be restored after the impoundment refills and the hydrology of the impounded system returns to supporting the wetland. This drawdown period may provide an opportunity for removing invasive plants; desired wetland plants can be replanted.

If a dam is removed, the wetlands adjacent to the former impoundment may disappear. An analysis of pre-dam wetland conditions will help estimate the likely affect of drawdown on wetlands. Original survey notes, aerial photographs, and land descriptions can help describe the pre-dam condition and size of wetlands. Wetlands are likely to reappear along the river channel in areas where they naturally occurred before construction of the dam. It can take several years for these areas to once again become functional wetland ecosystems. The total area of wetlands after dam removal may be more or less, depending on site-specific factors. Likewise, the wetlands next to the river may be different from those that were next to the impoundment. Because of changes in the water level, the types of plants and animals in the wetlands may change.
 
 

Next section: Morphology and sediment

URL: http://www.ies.wisc.edu/research/wrm00/educwet.htm