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in the lake are adfluvial; that is, they reside in the lake, but migrate upstream into tributaries to spawn (cf. bull charr in Figure 3). Hence, the fishes constitute an upstream feedback loop and enhance the ecological connectivity of the ecosystem (Figure 5). Ecological connectivity of the Flathead system is of course maintained in a quasi-equilibrium status by natural disturbance events (Figure 5). For example, the catchment is naturally disturbed by floods. Catchment hydrology is annually dominated by spring snowmelt and in that sense the hydrograph is very predictable. But, the magnitude of the spring spate is highly unpredictable, based on a 90 year period of record. Climatic events alternately juxtapose either continental (cold, dry) or Pacific maritime (warm, wet) air masses over the catchment, which determines precipitation patterns. Infrequently and under the extreme moisture conditions in the Pacific front, the two air masses collide directly over the catchment resulting in intense precipitation. Intermediate levels (10 - 20X mean annual flow) of flooding occur on about a 10 year return pattern and almost always during spring; but, high magnitude (20 - 50X mean annual flow) floods have occurred 17 times during the historical record. The timing and duration of high-magnitude floods and other extreme climatic events (Figure 5) is stochastic. Another example concerns the occurrence of wildfires caused by lightening strikes during dry periods. Mosaics of successional stages in forest stands characterize the uplands of the catchment as a result of these randomly distributed burns over many decades. Thus, natural disturbance is a fundamental feature of this ecosystem and, coupled with the zoogeographic history of the area, is responsible for the generally high biodiversity of plants and animals by preventing dominance by a few species. However, four generalized classes of human perturbations clearly have affected the natural attributes of this catchment: 1) stream regulation; 2) eutrophication; 3) food web manipulation; and, 4) erosion (Figure 5). While localized effects may vary and the magnitude of the impacts has not been so severe as to completely compromise ecosystem connectivity, anthropogenic disturbances have degraded natural structure and function. Stream R a lu ation Two large hydroelectric and flood control dams partially regulate flows in the mainstem river and volume in Flathead Lake (Figure 5). The spring flood pulse of the Flathead River is predominantly stored behind these dams and discharged during the base flow period. Owing to the presence of a natural bedrock sill at the outlet, the backshore of Flathead Lake historically was inundated up to ca. elevation 882.5 m (above sea level) during the spring spate; however, the lake returned to base level 11