North and Dry Falls Dams, two rock-faced, earthfill dams at the north and south ends of the Ice-Age channel of the Columbia River, now known as the Grand Coulee, form Banks Lake, the equalizing reservoir. This 27-mile-long reservoir, with an active storage capacity of 715,000 acre-feet, feeds Columbia River water into the Main Canal. In addition, it provides water on a return flow basis to produce power when the pump generating units are operating in the generating mode.
Major features forming and serving Banks Lake are the feeder canal with a capacity of 26,000 cubic feet per second, North Dam, 2 miles southwest of Grand Coulee Dam, and Dry Falls Dam and Main Canal headworks near Coulee City, 29 miles south of Grand Coulee Dam.
In conjunction with the addition of the six pump/generating units, the canal size was increased. The south side of the canal was removed, the base widened from 50 feet to 80 feet, an entire new south wall constructed, 8 feet added to the top of the north wall, and a new flume section was added to bypass a duplex tunnel cut-and-cover section. This increased the operating capacity to 26,000 cubic feet per second. Reconstruction was completed in 1981.
Dry Falls Dam is located 43 km within the area underlain by the massive basalt flows of the Columbia Plateau lava. The flows dip gently to the north under most of the foundation, but rise to the west in a steep monocline. Jointing is closely spaced but tight in the flows. Fractures are irregular and wide spaced even in the rocks of the monocline. Interflow zones of various widths separate the flows. The site is in a wide, steep-walled valley cut in basalt. On a large scale, the floor appears quite flat, but in detail narrow channels as much as 18m deep are present and numerous rock knobs and closed basins dot the floor. Except in the channels where thick deposits of silt are present overburden is thin and swamp deposits cover many low areas. Except where zone 2 and 3 of the dam overlie deep channels all structures are founded on basalt. Deep layers of silt were left under zones 2 and 3 in the channel areas. At the damsite the rock has exceptional bearing strength and vertical permeability is very low. Horizontal permeability is low within the mass of the flow and ranges from low to high in the interflow zones. High water tables in the reservoir walls limit losses through the interflow zones.