McNary Lock and Dam is located on the Columbia River in Oregon and Washington about 292 miles above the river mouth. The dam is approximately 7500 feet long with a maximum headwater to tail water of 92 feet. Following the current economic principles of construction, McNary Dam is built of two types of material. The powerhouse, spillway, nonoverflow, and navigation lock are constructed of concrete, while the connections to the abutments are constructed of soil materials. Both the concrete structures and the earth embankments are founded on basalt or basaltic flow breccia rock. The plan of the dam is shown in Figure 1. Foundation Treatment for Earth Dams on Rock, by Mr. Thomas F. Thompson, published by the American Society of Civil Engineers, volume 80, Separate no. 548, describes detailed foundation procedures and cites several of the foundation problems at McNary Dam. Geological Features at McNary Dam, by Mr. Charles J. Monahan, describes the layout of the dam and the geologic features of the dam site.