Crescent Lake National Wildlife Refuge
Mountain-Prairie Region


Wetland Values

Wetlands are so diverse that it is difficult for the scientific and regulatory communities to agree on a definition of the term. However, a simple definition is that wetlands are transitional areas between land and water, they are covered by shallow water or waterlogged soils, have soil lacking in oxygen, and grow water-loving plants. A wetland's abilities to absorb the force of floods, filter pollutants from runoff, recharge and discharge groundwater, and meter out water to streams and rivers are part of its hydrological cycle.

Hydrological factors such as water depth, frequency and duration of flooding, and the amount of dissolved or suspended materials determine a wetland's functions and plant and animal populations. Other hydrological factors that affect how fast water enters and the amount of sediment it carries are partly responsible for a wetland's shape, size, depth, and even location.

Wetlands moderate the flow of nutrients and sediment running off the land and regulate the flow of water moving downstream. They clean water by absorbing and filtering pollutants that would end up in lakes, rivers, and groundwater. They buffer riverbanks and lake shores from the eroding effects of currents and waves. By absorbing spring runoff, they take the punch out of floodwaters. Wetlands produce as much plant and animal life as do similarly sized areas of rain forest. Wetlands affect, and are affected by, the forests, prairies, lakes, streams, and rivers around them. Movement of wildlife, water, and nutrients is critical for sustaining all ecosystems, including wetlands (Minnesota Audubon Council, 1993).

Wetland complexes, which include a variety of wetland types, are crucial to many wildlife species and support a wide range of biological diversity. Many wildlife species can only find all the diverse food and habitats they need by using a combination of wetlands and other ecosystems. For example, seasonal and temporary wetlands warm early in the spring, and provide food for waterfowl at a time when larger, deeper wetlands are still frozen. They provide ideal feeding sites in terms of abundance and availability of high-protein foods throughout the spring and early summer. As they dry up, feeding shifts to semi-permanent ponds and lakes where adult insects are beginning to emerge (Swanson et al. 1974).

To learn more about the status of wetlands in the United States, see the National Wetlands Inventory Web Site.

Wetland Restoration

Restoring wetlands is often simply a matter of destroying manmade drainage facilities and letting water and vegetation return to the site naturally. The same tools used to destroy wetlands are used to restore them. If a wetland is drained by underground tile, a section of the tile is uncovered, removed, and the area is backfilled with soil. Wetlands that are drained by ditches are restored by the construction of an earthen ditch plug at the outlet to the wetland. A pipe or earthen spillway is used to regulate water levels and provide for overflow. On larger wetlands, a water control structure may be installed that allows a periodic drawdown of the wetland in an attempt to mimic the natural wet-dry cycle of many basins.

After restoration, wetland plants begin to reappear. The seeds of some wetland species can lie dormant, but viable, for as long as thirty years, waiting for moist conditions to return. These species will be among the first to appear after restoration, often even after years of agricultural herbicide use. Other seeds are carried into the wetland basin by birds, wind, and floods. It is generally not worth the cost to attempt to revegetate a wetland; if the conditions are right for a species, it will find its own way into the restored basin.

If you are interested in restoring a wetland on your property, check out our Partners for Fish and Wildlife Program.

Moist Soil Management

When mud flats are exposed by summer drawdowns of water levels, moist soil plants develop. These plants have the potential to produce high seed yields that serve as an important food source for waterfowl and other wildlife. When compared with grain from agricultural fields, moist soil plants may provide waterfowl with lower levels of metabolizable energy, but they contain a better balance of nutrients (Fredrickson and Taylor 1982, Fredrickson 1991, Havera 1992).

A diversity of invertebrate and vegetative foods from different wetland types are needed on fall and spring migration areas to meet the changing nutritional demands of waterfowl and provide them with a complete diet. Although mallards like corn, studies show that if they are fed only corn, even when supplemented with oyster shells to provide calcium and grit, they begin to die after about 60 days (Havera, 1996).

Many restored or enhanced wetlands are equipped with stop log structures that allow the manipulation of water levels. This manipulation permits us to control the drawdown of water levels in a way that provides optimum moist soil plant growth. Water levels are slowly increased in the fall to provide the desired mix of open water and mature flooded moist soil vegetation.


Fredrickson, L.H. 1991. Strategies for water level manipulations in moist-soil systems. Waterfowl Management Handbook, Fish and Wildlife Leaflet 13.4.6. 8 pp.

Fredrickson, L.H. and T.S. Taylor. 1982. Management of seasonally flooded impoundments for wildlife. Fish and Wildlife Service Resource Publication 148. 29 pp.

Havera, S.P. 1992. Waterfowl of Illinois: status and management. Final report to Illinois Department of Conservation. W-110-R-2. 1,035 pp.

Havera, S.P. (Illinois Natural History Survey). November 1, 1996. Personal Communication with Susan B. Julison.

Minnesota Audubon Council. 1993. Minnesota Wetlands: A primer on their nature and function. Ed. Cheryl Miller. Saint Paul, Minnesota. 23 pp.

Swanson, George A., M.I. Meyer, and J.R. Serie. 1974. Feeding ecology of breeding blue-winged teals. Journal of Wildlife Management. 38(3):396-407

Last updated: March 21, 2011