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and associated interactions between natural and human disturbances complicates resource management within the ecosystem . The threat of deteriorating water quality in Flathead Lake from urban sewage, the proposed Canadian coal mine and burgeoning road building and clearcutting on Federal and private timber lands stimulated management actions designed to implement conservation of natural conditions in the tributaries and to reduce nutrient loading in the lake by about 20% (i.e., to near natural conditions). Actions included a ban on the sale of phosphorus-containing detergents (mandated by the Montana State legislature), construction of new sewage treatment plants to allow phosphorus and nitrogen removal from all urban effluents in the catchment and voluntary imposition of "best management practices" (BMPs) to reduce nonpoint sources of nutrients, especially those associated with accelerated erosion in the catchment (mandated by the State Water Quality Bureau, which has statutory authority to enforce water quality laws). During 1983-1990, annual nutrient loads into the lake decreased (least squares regression, P< .1, J. Stanford and B. Ellis unpublished); and, as noted above, Anabaena blooms did not reoccur. This of course suggested that initial management actions were successful. However, construction of a new sewage treatment plant for Kalispell, Montana, which has been the largest point source of bioavailable nutrients in the past, is not yet complete. Moreover, very little, if any, of the reduced nutrient load can currently be related to voluntary BMPs because their utility in improving water quality has not been quantified empirically in the Flathead Basin. The apparent reduction in nutrient loading and lack of reoccurring Anabaena blooms may be due to at least three other interactive linkages. First, catchment precipitation has been below average since 1983. Natural loading rates of water and nutrients have been generally lower on an annual basis than occurred earlier in the period of record. However, average concentrations in the river did not change significantly. Second, operations at Hungry Horse Dam changed from primarily mid-winter to summer and fall discharges, in response to economic considerations for hydropower production as related to demands for higher summer flows in the lower Columbia River to more effectively flush smolts of anadromous salmon out to sea (discussed below). Owing to thermal stratification in the reservoir and the hypolimnial (bottom) release mode of the dam, the high volume water masses from Hungry Horse Reservoir are cold (4 - 70 C) and dense relative to ambient temperatures (unregulated, natural) in the river below the dam and within the epilimnion (surface) of Flathead Lake, which is also 16