From a watershed perspective, riparian habitats in particular have come to be seem as diverse and essential habitats for many organisms and processes, and they provide a critical link between aquatic and upland ecosystems. Riparian vegetation, for instance, controls much of the environmental regime of stream ecosystems (although this is less true of larger streams and rivers), and plays a vital role in determining the quantity and timing of flows as well as stream temperature, which is strongly influenced by shadows cast by riparian vegetation. Riparian zones are the source of woody debris, an extremely important structural component of the aquatic ecosystem; such debris creates a structural complexity vital to the stream's ability to store sediments, detain water, and create a variety of specialized habitats supporting biological diversity. Wetlands play similar important roles in maintaining biodiversity and watershed processes.
Watershed management can be used as a tool to enhance wildlife objectives, one element of ecological diversity. Although not relevant in all cases, watershed planning can include attempts to avoid degradation of critical wildlife habitat or restore habitat lost to past decisions. For example, the current phase of relicensing nonfederal hydropower facilities now gives attention to restoring minimum water flows below dams where appropriate, thus restoring fish habitat. Communities rehabilitating waterways through urban areas often attempt to restore fish habitat, for instance by placing logs or other debris in strategic locations to serve as cover for fish.
The artificial introduction of exotic species, in some cases for the express purposes of manipulating watershed processes such as erosion, can have a negative effect on ecological diversity. Kudzu and water hyacinths in the southeastern United States, tamarisk in Western states, purple loosetrife in New England, arrundo in California, and Russian olive in many sections of the country were well-intentioned introductions that have escaped control and now infest many watersheds in undesirable densities, eliminating native species and reducing overall diversity.
Increased attention to diversity and the related issue of habitat protection often calls for making hard decisions about trade-offs involving other watershed benefits—for instance, accepting some lost power opportunities in exchange for increased flows below dams.
Overall the threat to aquatic biodiversity in North American is great. In the United States, aquatic organisms are among the nation's most imperiled (Table 1.1). Of the entire flora and fauna, the four groups with the greatest percentage of species currently extinct or at risk of extinction are aquatic.
Our nation's historical approach to managing of watershed resources has reduced the populations of native fishes in American streams. New England mill dams eliminated shad runs on many streams by the early 1800s, and two thirds of