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(878.8 m.a.s.l.) by mid-July. Kerr Dam was built downstream from the sill in 1937 and extends the full pool (881.8 m.a.s.l.) period into late October to facilitate hydropower production. Hungry Horse Reservoir was first filled in 1953 and stores runoff from the entire South Fork subcatchment. Hydropower operations currently cause both flow and temperature problems in the river segments downstream from the dams. The varial zone of the river channel (Figure 1) is alternately inundated and dewatered by fluctuating flows related to power production below the dams. As a consequence, the varial zone is quite large and is essentially devoid of aquatic biota. Sluicing of the substratum by clear water flows has removed the smaller particles leaving larger rocks and cobblestones firmly implanted on the river bottom (a phenomenon of regulated rivers known as armoring, Simons 1979). Capture of flood flows has partially or totally eliminated the natural fluvial disturbances on the floodplains of regulated river segments, thereby allowing senescence or other alteration of riparian plant communities. Since Hungry Horse Dam discharges from the bottom of the reservoir, nutrient concentrations are elevated relative to the free-flowing river segments and algal mats coat the armored substratum in the minimum flow channel below the varial zone. Stream regulation has reduced the biodiversity in the dam tailwaters by about 80%. Spawning, juvenile recruitment and growth of resident and adfluvial fishes has also been seriously compromised by extension of the varial zone in both regulated river segments; and, cold (5-80 C) summer temperatures in the effluent water from Hungry Horse reservoir compound the problem in the mainstem river upstream from Flathead Lake (Stanford in press). Eutro i ion Plant growth in most of the lakes and streams of the Flathead catchment is limited by a general lack of labile nutrients. Most of the waters appear to be phosphorus limited or co-limited by paucity of both nitrogen and phosphorus (Dodds and Priscu 1989). Many alpine and subalpine lakes contain no measurable soluble reactive phosphorus and < 20 ug/L nitrate, owing to the lack of these minerals in the Precambrian argillites that dominate the bedrock of much of the catchment. Therefore, bioproduction in these waters is very low (Stanford and Ellis 1988, Stanford and Prescott 1988). Consequently, abnormally accelerated algal production associated with anthropogenic nutrient enrichment (i.e., eutrophication) is a primary concern, particularly as it relates to degradation of the high quality water in Flathead Lake. Seventeen percent of the total bioavailable phosphorus load entering Flathead Lake annually is derived from sewage treatment plants in the catchment. Thirty percent of 12