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Eight damsites have been mapped on the Flathead River between river mile 3 and Kerr Dam at mile 72. The river, between miles 0 and 25, flows west across a west-northwestward-trending mountain range; Knowles and Perma damsites are in this section. The other six sites are between mile 25 and Kerr Dam; in this distance the river flows southward along the western edge of the Mission Valley. The rocks involved in the abutments and foundations of the damsites include hard fine-grained gray to light-gray quartzite, sandy argillite, greenishgray argillite, diorite, and some metamorphosed argillites of the Precambrian Prichard Formation and Ravalli Group. At places along the edge of the Mission Valley, the Precambrian is overlain by Tertiary (?) weathered talus breccia, conglomerate, sandstone, siltstone, a reddish-brown gritty clay or microbreccia, and volcanic tuff. Overlying the Tertiary (?) and the Precambrian are Pleistocene till, glacial-lake-bed silts, outwash gravel and sand, and Recent alluvium. Knowles and Perma damsites, near river miles 3.1 and 11 respectively, are alternates for developing the river either by a high dam that would have a flow line at altitude 2,705 feet and back water to Kerr Dam or by run-of-theriver dams in combination with other upstream dams. At Knowles damsite and downstream, the river has eroded a deep valley along the crest of a gentle anticline. The Prichard Formation is exposed in the valley walls, and glacial-lake-bed silts and sand have filled the lower part of the valley to a depth of 255 feet. Artesian water occurs in the valley fill. The site is feasible only for an earthfill dam. A large rock knob, near the center of the valley, is connected to the north wall by a low saddle and offers an excellent foundation for powerhouse and spillway sites. At Perma damsite sections C-C' at mile 10.9, only 160 feet upstream from Perma Bridge, and D-D' at mile 11.3 are possible axes for a dam that would have a flow line between 2,600 and 2,705 feet. The abutments are in strong rock that would be good foundation sites for the appurtenant structures. At C-C', a wide terrace south of the river is underlain by 23-56 feet of pervious sand and gravel. Two spillway sites are on D-D'; one is across the rock spur forming the right abutment; the other is along an abandoned high-level channel of Flathead River that swings 2,200 feet south of the left abutment. For a high dam, geological conditions are slightly better at D-D' than at C-C'. D-D' and E-E', at mile 11.9, are possible axes for a dam having a flow line

Geology of damsites on Flathead River, mouth to Flathead Lake, Lake and Sanders Counties, Montana