Lake Roosevelt lies in a semi-arid zone: precipitation ranges from about 10 inches per year at the southern end to about 20 inches per year at the north end.
Lake Roosevelt pumps 2.5 million acre feet (or over 814 billion gallons of water into Banks Lake annually. Water from Banks Lake irrigates approximately 550,000 acres of farmland each year.
The Lake Roosevelt watershed drains 44,969 square miles, almost 88% of those 44,969 square miles are in Canada.
The water in Lake Roosevelt comes from glacial ice, lakes and precipitation (mostly melting snow pack in the spring). Close to 89% of Lake Roosevelt’s water comes from the Columbia River, 7% comes from the Spokane River and 4% comes from a combination of the Colville, Kettle and Sanpoil Rivers.
Lake Roosevelt is over 150 miles long and provides over 630 miles of shoreline.
Lake Roosevelt stores up to 9.4 million acre feet of water behind Grand Coulee Dam.
Full pool (when the reservoir is filled to capacity) at Lake Roosevelt is 1290 feet, this is the elevation of the surface of the water above sea level. The elevation level of the lake can fluctuate as much as 80 feet over the course of a year. At full pool (elevation of 1290 feet) the surface of Lake Roosevelt covers close to 81,000 acres.
The water depth of the lake varies from approximately 400 feet deep just upstream from Grand Coulee Dam to about 14 feet deep just below the Canadian border.
Water temperatures in Lake Roosevelt are around 60 degrees Fahrenheit in June. There is very little temperature variation over the summer.