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\.. \ 86 . .. Stream Ecosystem StabiJ.ity ConcP:E! (Stream ecosystem stability is a result of energy flow and the, self-regulating (homeostatic) mechanisms that bring about relatively I constant conditions. When a stream ecosystem is changed from a stable 1 state to an unstable state by an uncontrollable catastrophic influence such \ as flood, fire, or man-caused perturbation such as channelization, 1 l self-regulating mechanisms such as benthic organic matter retention tend to i bring about a return to constancy. Small stream ecosystems in Ilatural watersheds in Idaho, Michigan, Oregon, and Pennsylvania tended to be more stable than medium-size streams (Minshall et al. 1983). Under natural, riparian vegetative-climax conditions, headwater feeder streams appear to maintain a more constant or predictable trophic resource, more favorable substrate, and more moderate and reliable water flow than intermediate streams because of the smaller streams' greater physical retentiveness. 1 Thus, small streams are highly resistant to change alld therefore stable. Of course, once degraded by man-caused activities such as fire, channelization, clear-cutting, clearing and snagging operations, etc. stability will be lost and the recovery perIod may range from short to long to never. In contrast to small streams, medium-size streams constantly undergo change as reflected by their wide range of discharges, large scale changes in bed movements, lack of storage and their attendant buffering capacity in terlllS of food resources (Minshall et al. 1983). Large rivers may show a tendency toward increased stability because of reduced water turbulence near the streambed, the tendency for smoothing out large upstream flow fluctuations and off-stream (flood plain) storage of water and materials. . Generalized Warmwater/Coldwater ~tream Characteristics Warmwater streams have been characterhecl as occurring at relatively low elevations, having cool to warm water (greater than 20 C) in summer, quiet flows, high turbidities, more pools and fewer riffles than co1d.ater streams, substrate of small particle size, rooted and floating vegetation, and sparse shade and cover (Winger 1981) (Ta.bl.e 3). W,a.rmwater streams generally contain more introduced species and have mor,= manIllade modifications (Moyle and Nichols 1973). Goldwater streams have been characteri~ed as occurring at relativel.y high elevations and having cool to cold wat,=r (less than 20 C) in summer, steep gradient, turbulent flows, low turbidites, more :eif f les and fewer pools than warmwater streams, substrate of large particle size (gravel, rubble, boulder), extensive periphyton and low rooted and floating vegetation, extensive shade and cover (Table 4). Coldwater strea~s generally contain lower species diversity and have few'~r rnanmade modifications.