Abstract: The various components of inflow and outflow to and from the lake and their relative magnitudes were identified. In 1972 water year the total inflow to the lake was 200,000 acre-feet, but it was only 75,000 acre-feet in 1973 water year. In 1972 water year the Donner and Blitzen River contributed 55 percent of the inflow, with Silvies River, direct precipitation, and Sodhouse Spring contributing 28, 13, and 4 percent respectively. Om 1973 the Donner and Blitzen River contributed 61 percent of the inflow. The Silvies River, direct precipitation, and Sodhouse Spring contributed 1, 25, and 12 percent respectively. Ground-water inflow, other than Sodhouse Spring, appears to be negligible. In 1972 water year, 81 percent of the outflow was by evapotranspiration and 19 percent was surface outflow through The Narrows. In 1973, 96 percent of the outflow was by evapotranspiration and 4 percent was surface outflow through The Narrows. Ground-water outflow is negligible.