Valley fill in the area is as thick as 120 feet or more. The fill consists of coalescing stream deposits, alluvial fan material, and in the upper portions, variable thickness of glacial outwash fan material. A considerable portion of the slopes above the valley are covered with slope wash. A small northwest-southeast fault was mapped between the upper and lower axes during preliminary exploration. The fault is located about 100 feet south of the downstream toe of the dam.
Records indicate that rock on either side of the fault is sheared in the left abutment.