A comprehensive investigation of the physical, chemical and biological limnology of Bighorn Lake (formerly Yellowtail Reservoir) and its tributaries was initiated in the fall of 1967 to determine if a decline in primary productivity of this new impoundment would occur. The study specifically encompassed the heat budget, salinity regime, internal currents and biological productivity (phytoplankton) of the reservoir. The three-year study was undertaken to relate the physical and chemical environment of the reservoir to primary production and to determine what changes in the primary production took place over the entire study. An effort was also made to assess the physical and chemical characteristics of the influent and effluent waters of the reservoir.