systems. However, the habitat value of restored coastal wetlands is not fully documented.
Stream and riverine wetlands are often severely altered. Not only have rivers been the depository for most liquid pollutants, but their hydrologic regimes have also been altered by dams, pumping, dikes, channelization, dredging, bank stabilization, and watershed development. Wetlands in headwater areas, in oxbows, and in low-velocity channels have not been extensively restored, although rather largescale floodplain forest restoration programs are proposed for areas in the lower Mississippi. Many small-scale restoration projects have been undertaken as part of local greenway and stream restoration programs such as the Urban Streams program in California.
Efforts to restore riverine wetlands are complicated by the hydrologic and sediment regime changes typical of most rivers, which make it impossible to return wetlands to their natural condition without massive removal of dams, channelization, and so on. Nevertheless, these wetlands are increasingly recognized as having great value for water quality protection, fish and wildlife habitat, flood control, and bank stabilization.
Many small, depressional wetlands were formed by glaciers 9,000 to 12,000 years ago. Hundreds of thousands of these wetlands exist in the northern tier of states (prairie potholes). Other depressional wetlands were created by solution (karst topography), the wind (sand hills of Nebraska), or other processes (Carolina bays of the South-east). These wetlands have not, in most states, been modified as substantially as have riverine wetlands. However, many have been drained or are used during dry years for agriculture. Others have been used as landfills or filled for urban development.
Except for efforts to remove drainage tiles and restore natural drainages in the prairie pothole region, few efforts have been made to restore such depressional wetlands. Restoration efforts may be quite inexpensive if cessation of agriculture or blockage of drainage ditches is the primary activity. However, restoration is potentially expensive where fill must be removed or where extensive removal of drainage tiles is involved.