Geology of damsites on the upper tributaries of the Columbia River in Idaho and Montana - Knowles and Perma damsites, lower Flathead River, Sanders County, Montana

Knowles and Perma damsites, at river miles 3 and 11 respectively, are alternates for developing the lower Flathead River. A high dam with a flow line at altitude 2,705 feet would back water to Kerr Dam at river mile 72 and would produce a hydraulic head of 235 feet at Knowles and 225 at Perma. Low head dams in combination with upstream dams could be used to develop the sites also. Economic studies of both development schemes should be made.

Precambrian metasedimentary rocks including quartzites, impure quartzites, and argillites, of the Prichard formation, Belt series, make up the valley walls. At Knowles the river is flowing along the crest of a gentle anticline. At Perma the river is flowing across the nose of a southeast-plunging asymmetrical fold well outlined by two large diorite sills. At both sites the valley bottom is filled with about 255 feet of interbedded very fine-grained sand, silt, and clay deposited in glacial lakes. Either site is feasible only for an earthfill dam. Additional testing of the unconsolidated valley fill is necessary.

At Knowles damsite a rock knob 800 to 1,600 feet south of the right valley wall extends through the valley fill and rises to slightly above 2,650 feet. Either line A-A', which crosses the knob, or B-B', 400 feet upstream, is suitable for a high dam. B-B' has the smaller valley cross section. A-A' has a narrower section underlain by lake beds and offers a wider choice of sites for appurtenant works. A-A' is the best axis for a low dam and most likely for a high dan too.

At Perma damsite sections C-C' and D-D' are possible axes for a dam with 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 to 5o feet of pervious sand and gravel. Contact between the gravel and underlying lake beds is between altitude 2,444-2,480 feet. Two spillway sites are present 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' are possible axes for a darn with a flow line between altitudes 514 and 2,600 feet. At either axis the foundation and right abutment would be unconsolidated lake beds. At D-D', the left abutment is the Prichard formation and at E-E' a diorite sill. The sill would be an excellent foundation for appurtenant structures. Geologically, E-E' is the best axis for a low dam.