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North Dam

North Dam
 
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Columbia Basin Project
 
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North Dam is located  2 miles southwest of Grand Coulee Dam. 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 cfs, 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.

Geology

North Dam is located on the northern edge of the area underlain by the Columbia Plateau lava. The structure is in the center of a deep, relatively flat bottomed, steep-walled canyon about 1.6 km downstream from the Columbia River gorge.  Basalt flows and interbedded Latah beds underlie the northern part of the dam.  The Latah is a lake deposit of clayey to sandy sediment that developed along the margin of the basalt.  In the area where the southern part of the dam is located, granite and metasediments of the basement rock formation come to the surface. In the deepest part of the canyon excavation, alluvial deposits of silt, sand, and gravel as well as Lake Beds of Nespelem silt and sand cover the older rocks.  Surface deposits of swamp muck and blow sand form a thin surface mantle. At the north end of the dam an isolated block of basalt rises above crest elevation. This basalt, together with a considerable amount of clay and sandstone of the Latah Formation, is interpreted in the exploration reports as slide material.  The movement is believed to have taken place near the end of the canyon cutting cycle during the ice age.  No movement of the block or surrounding material has been noted during construction or operation of the dam.  In the north part of the dam excavation, the trench reached basalt of Latah beds. In some places these materials were in place and in others classified as "disturbed" material. Both types seemed suitable for a dam foundation. On the south end the trench was carried down to granite. The rock surface was irregular and the depth of the cut varied from place to place. The resulting foundation was satisfactory.  Between these two areas the recent alluvial fill was deep and a layer of materials such as gravel and Nespelem lake beds was left beneath the outer zones of the dam. Such materials as swamp muck or blow sand were entirely removed. This formed a stable foundation and no excessive settlement or movement has been recorded.


Last updated: Jun 21, 2012