held accountable for such long-term, uncontrollable factors. However, watershed assessment and prioritization provide a framework for identifying and avoiding future high-risk areas for mitigation sites. The contingency of climate change and sea-level rise argues for landscape-scale planning and implementation of wetland creation, restoration, and preservation.

WATERSHED-SCALE PATTERNS OF WETLAND LOSSES

As discussed in Chapter 1, wetland losses have occurred with changes in runoff and erosion due to urbanization and agricultural land uses. Other factors that result in both direct fill/destruction and indirect impacts and wetland losses include channelization, groundwater withdrawal, and flood-control practices.

Losses Due to Urbanization

Urbanization of watersheds is often extensive in headwater regions. In older, built-out urban areas, headwater wetlands and wetlands along first-order streams may have been put into storm sewer networks. This loss of streams and springs is well documented in some regions (e.g., Williams 1977). The loss of wetlands in this context can be evaluated by a comparison of wetlands and their distribution in urban and adjacent nonurban watersheds.

Losses Due to Agricultural Uses

The position of the stream channel head in the landscape is controlled by runoff processes and surface topography (Dietrich et al. 1986). An increase in overland flow tends to move a stream channel upslope because less area is required to initiate the channel head. This upslope migration of stream channels has been documented in agricultural areas and has often resulted in the loss of headwater wetlands and some first-order stream wetlands. Thus, the pattern of wetlands in a watershed often reflects previous land-use practices. For example, channel incision in Wisconsin, Maryland, and Pennsylvania has resulted in significant loss of wetlands in headwater positions and along first-order streams (Prestegaard 1986; Prestegaard and Matherne 1992).

Many wetlands have also been lost due to land drainage for agricultural or other land uses. For examples, sedge meadows, wet prairies, and other wetlands were easily drained for agriculture in central Wisconsin (Curtis 1959), Iowa, and elsewhere in the Midwest (Prince 1997). In this process, unchannelized portions of the landscape are channelized into



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