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thermally stratified in the summer (220 C surface, 40 bottom). Thus, summer discharges from Hungry Horse Dam essentially dilute the pollutants entering from the urban and argricultural areas in the Kalispell Valley. Moreover, these cold waters and the nutrient load immediately sink to the lake bottom (underflow) upon entry into Flathead Lake. Since the lake is maintained at full pool during the summer for ease of access by boaters, Kerr Dam must discharge water volumes equal -to the inflowing volumes. But, Kerr Dam releases water from the surface layers of the lake, owing to its location below the natural outlet sill. The net effect on the limnology of the lake appears to be 1) a significant reduction in the heat budget, 2) cooler surface temperatures during the plant growing season, 3) stripping of plankton and nutrients from the surface by the Kerr withdrawal current and 4) deposition of a large portion of the summer and fall nutrient load far below the upper portion of the water column that is penetrated by sunlight. Therefore, conditions favorable for sustained algal biomass, especially forms like Anabaena, in the epilimnion of the lake may have been compromised by hydropower operations. Third, food web shifts caused by the collapse of the kokanee fishery may have influenced grazing rates on the algae. Owing to intense predation by Mysis, zooplankton biomass decreased almost an order of magnitude in the peak Mysis years, 1987-1988, compared to pre-Mysis measures. During 1988-1990, Mysis numbers decreased from 125 m-2 in 1987 to 30 m-2 in 1989 (Spencer et al. 1991) and 35 m-2 in 1990 (Spencer 1991), owing to predation by bottom-oriented fishes (whitefishes (Coregonus spp.), lake trout, and bull charr). Phytoplankton primary production was the highest on record in 1988 and decreased during 1989-1991 in concert with declining Mysis numbers. At the same time, cladoceran zooplankton have recovered during periods of thermal stratification. Apparently, large numbers of Mysis do not penetrate the thermocline and enter the epilimnion during summer. This thermal refugia from Mysis and the lack of kokanee or other surface-dwelling planktivores apparently has allowed Daphnia thorata to increase and the inference is that grazing on phytoplankton has also increased (Stanford et al. 1990). These interpretations are based on preliminary analyses of long-term trends in the various data bases for Flathead Lake. Our main point here is simply to reinforce by example the idea that food web dynamics in lakes can be strongly interactive in response to both bottom-up (nutrient supply) and top-down (Mysis introduction) effects (see also Carpenter 1988). Interactions between dam operations, natural circulation patterns and shoreline erosion in Flathead Lake are also noteworthy. It is exceedingly difficult to move large water masses through Flathead Lake while also maintaining it at full pool elevation. 17