Both North and Dry Falls Dams are rock-faced, earthfill dams. They were built to create Banks Lake, 27 miles (43 km) long as an equalizing reservoir, with storage capacity of 715,000 acre-feet for irrigation. The reverse flow, back to the Columbia River, provides additional electrical power.[5] The feeder canal from Banks Lake has a capacity of 26,000 cubic feet per second, Dry Falls Dam and the Main Canal headworks are near Coulee City. While not a part of the Dry Falls Dam, the Main Canal Headworks was expanded to add a six pump/generating units. The Main Canal was widened from 50 feet (15 m) to 80 feet (24 m).[5]
History
Originally known as the South Coulee Dam, Dry Falls Dam began construction in 1946. Roy L. Blair and Company and James Crick and Sons, were awarded the contract for $2,771,887 to build the dam. Work began in mid-July. Included in the contract was the dam across the Grand Coulee, the approach channel, the headworks and control structure, and the initial reach to the main canal.[5] A cutoff trench was created down to bedrock along the length of the dam. A continuous concrete wall was centered in the trench with footings at least 3 feet (0.91 m) deep. The wall prevents seepage of water under the dam, and is from 5 to 10 feet (1.5 to 3.0 m) high.[5] The construction of the canal head works began in March 1948, and was completed a year later. Designed as a zoned earth embankment structure with an impervious central core, a semi-pervious layer on either side of the core, and a layer of rock fill. The semi-pervious layer was built in 1 foot (0.30 m) layers and compacted. The rock fill laid up in 3 feet (0.91 m) layers without compaction.[5]
