Grand Coulee Dam is one of the largest concrete structures in the world. This dam, which raises the water surface 350 feet above the old riverbed, is 5,223 feet long, 550 feet high, and contains about 12 million cubic yards of concrete. The original dam was modified for the Third Power Plant by construction of a 1,170-foot-long, 201-foot-high forebay dam along the right abutment approximately parallel to the river and at an angle of 64 degrees to the axis of Grand Coulee Dam. The total length of the main dam, forebay dam, and wing dam is 5,223 feet. The spillway of the dam is controlled by 11 drum gates, each 135 feet long, and is capable of spilling 1 million cubic feet of water per second with Lake Roosevelt at fullpool (1290.0 feet above sea level). The dam also contains forty 102- inch-diameter outlet tubes. Within the dam are 8.5 miles of inspection galleries and 2.5 miles of shafts.
Franklin D. Roosevelt Lake, the reservoir behind the dam, extends 151 miles northeast to the Canadian border and up the Spokane River, a tributary of the Columbia, to within 37 miles of Spokane. The total storage capacity of the reservoir is about 9.6 million acre-feet, and the active capacity is about 5.2 million acre-feet.
The average discharge at Grand Coulee over a period of years is approximately 110,000 cubic feet per second. On June 12, 1948, during an historic Columbia River flood period, the maximum discharge (turbine and spill) recorded was 637,800 cubic feet per second. The annual volume inflow has varied from a minimum of 48.5 million acre-feet to a maximum of 111.8 million acre-feet. The average annual inflow to Lake Roosevelt is 99.3 million acre-feet. The April through July inflow accounts for 65 to 70 percent of the total annual inflow volume.
Foundation: Hard, sound, massive granite varying from coarse-grained in right abutment to fine-grained porphyritic in left abutment.