Hells Canyon is the deepest canyon on the North American continent, and the Hells Canyon Project is located at one of the narrowest points in the canyon. Before crews could even begin construction, a 23-mile access road was cut along the Idaho side of the canyon (Now known as the Devil's Tail). Morrison-Knudsen, the contractor, set up trailer-type offices, a first aid facility, and machine and carpentry shops on a small strip of level ground two miles upstream. Housing and a mess hall for construction crews were built nine miles upstream.
The narrow canyon prevented crews from locating the switchyard adjacent to the powerhouse and generators, like most hydropower projects. Instead the switchyard was mounted on the dam's face. Helicopters helped move tools and equipment and were used to erect transmission towers. These towers support the transmission lines that carry electricity out of the canyon on the Oregon side.
Although construction crews had to work in a cramped and nearly inaccessible place, they built Hells Canyon Dam to full height in only six months. - source: Idaho Power.
The Seven Devils Mountains extend along the Idaho/Oregon border for roughly 40 miles between the Idaho towns of Whitebird and Council. They are bounded by the Snake River on the west and the Salmon and Little Salmon rivers on the east. They form the eastern wall of Hells Canyon. The range ranks high among Idaho's mountain chains in terms of ruggedness and scenic quality; it is the state's most precipitous range, with elevations varying from just above 1,000 feet at the Snake River to 9,393 feet on the summit of He Devil.