During the 21st century, climate change is expected to alter aquatic habitats throughout the Northern Rocky Mountains, intermountain basins, and western Great Plains. Particularly in montane watersheds, direct changes are likely to include warmer water temperatures, earlier snowmelt-driven runoff, earlier declines to summer baseflow, downhill movement of perennial channel initiation, and more-intermittent flows (see Chapter 4), as well as indirect changes attributable to altered and perhaps novel disturbance regimes. For animals restricted to freshwater aquatic environments for most or all of their lives - fishes, amphibians, crayfish, mussels, and aquatic macroinvertebrates - changes in habitat and in hydrologic regimes are likely to lead to marked shifts in their abundance and distribution. This is primarily because many of these species are ectothermic (cold blooded); thus, environmental conditions dictate their metabolic rates and nearly every aspect of their life stages, including growth rate, migration patterns, reproduction, and mortality (Magnuson et al. 1979).