18: Watershed Level Approaches
Frank J. Rahel and Donald A. Jackson
Assessing fish habitat requirements is a major focus of fisheries management and aquatic conservation efforts. Historically, we have tended to think of fish habitat in terms of local conditions such as water depth, current velocity, and cover. However, fish habitat can be viewed at a variety of spatial scales varying from microhabitat conditions to stream channel units or lake zones up to watershed level characteristics (Fausch et al. 2002). Furthermore, these scales form a hierarchy such that local habitat conditions often are the result of processes that operate at much larger spatial scales. For example, regional geology and glacial history can influence the productivity and morphology of lakes (Riera et al. 2000). In streams, basin shape and geology interact with riparian vegetation to determine the types of habitats present (Frissell et al. 1986; Modde et al. 1991). In describing how stream features are controlled by characteristics of the drainage basin, Hynes (1975) noted, “in every respect, the valley rules the stream.” This chapter describes approaches used to relate both large-scale habitat features and habitat patchiness to fish abundance patterns in lakes and streams.
A watershed is an area drained by surface and groundwater flow. A drainage basin is a watershed that collects and discharges its surface streamflow through one outlet or mouth. The term catchment refers to a subdrainage or the land area draining toward a specified point of interest within the drainage basin. Watersheds of large rivers are commonly called basins, such as the Missouri River basin or the Ohio River basin. Watersheds exist within a hierarchical framework, such that catchments exist within watersheds that, in turn, are part of basins.