existing watersheds, thus extending the stream network. Channelization of downstream portions of these river networks often deepens the existing river channel and destroys adjacent riparian zones and wetlands (Prestegaard et al. 1994). Inadvertent channel network changes have also occurred as a result of agricultural and urban land uses. Increased runoff from agricultural lands has generally caused a headward migration of stream channels in many areas. This leads to incised stream channels in many headwater regions (Costa 1975) and loss of headwater wetlands (McHugh 1989; Prestegaard and Matherne 1992). Thus, channelization practices have led to the loss of both prairie pothole wetlands that were not originally part of watershed systems and riparian wetlands along the original river courses.
Groundwater withdrawals have particularly affected wetlands and riparian zones along higher-order streams in arid and semiarid regions. An example is provided by Stromberg et al. (1996), who demonstrated the effect of groundwater withdrawals on riparian zones and riparian wetlands in arid regions.
Wetland losses have also occurred as a result of flood-control practices. For example, levees restrict connections between the river and the adjacent flood plain, affecting riparian wetlands. Levees, reservoirs, and other flood-control structures also serve to modify the timing of flood events, either by minimizing the size or modifying the frequency and duration of flood flows. Infrequent flooding can modify floristic communities in flood-plain areas, often allowing the development of forests in formerly herbaceous wetlands (Bren 1992). The importance of flooding, particularly in large (downstream) river systems, has been emphasized as the flood-pulse concept (Bayley 1995; Bornette and Amoros 1996; Middleton 1999).
Several authors have argued for a hydrogeological or hydrogeomorphic template for wetland mitigation and development (Moore and Bellamy 1974; Bedford 1996; 1999). This would suggest compensation projects that would be selected based on set functional priorities of the watershed. In practice, some in-lieu fee programs (see Chapter 4) have