A. HISTORY AND DESCRIPTION

1. General. Canyon Ferry Dam and Powerplant are concrete structures which, with other appurtenant works, comprise the Canyon Ferry unit of the Missouri River Basin project. Canyon Ferry Dam was constructed to provide regulation of runoff for power generation and to permit increased diversions in the upper Missouri River Basin upstream and downstream from the dam for irrigation of 305,500 irrigable acres of new land and for supplemental irrigation supplies for 187,700 acres, through replacement from storage to protect downstream water rights. Canyon Ferry Powerplant, with an installed capacity of 50,000 kilowatts, supplies expanding power needs for commercial use in a wide surrounding area by interconnection with existing Montana Power Company facilities. Electric energy generated at the powerplant during the two years 1955 and 1956 averaged 320,566,000 kilowatt-hours.

Irrigation and the production of low-cost power are two of the major benefits of the Missouri River Basin project. Other major potential benefits are control of floods, provision of water for municipal use, pollution abatement, silt control, creation of new recreational facilities, and enhancement of fish and wildlife resources.

2. Location and Description of Unit. Canyon Ferry Dam is located on the main stem of the Missouri River about 58 miles north of its head. Helena, the capital of the State of Montana, is 17 miles southwest of the dam site and can be reached over a county road. The railhead for the unit is at Louisville, about 12 miles east of Helena and about 10 miles southwest of the dam site. Louisville is also on Federal Highway No.

10N which parallels the Northern Pacific Railroad at this location. The Missouri River flows in a general northwesterly direction at the dam site as shown in figure 1. The dam and powerplant were constructed in a narrow constriction of the Missouri River Valley about 11⁄2 miles downstream from the old Canyon Ferry Dam and Powerplant of the Montana Power Company (fig. 2). This old 7,500-kilowatt powerplant, completed in 1898, was dismantled prior to the completion of the new Canyon Ferry Dam and Powerplant. The site of the new dam lies in the upper reach of Hauser Lake which is formed by an 18,000-kilowatt hydroelectric installation of the Montana Power Company (fig. 3).

The final design for Canyon Ferry Dam (fig. 4) provided for a concrete gravity structure approximately 1,000 feet long at crest elevation 3808.50, and with a maximum height of about 225 feet from the lowest foundation excavation. The dam has a maximum thickness of approximately 160 feet at the base of the dam in the maximum section. A 21-foot roadway crosses the dam, spanning the spillway overflow section by a structural steel bridge.

The spillway consists of an overflow section in the central portion of the dam with crest at elevation 3766.00, controlled by four 51- by 34.5foot radial gates. Four river outlets are provided in the spillway section of the dam, each being 84 inches in diameter controlled by 77inch regulating gates. A 156-inch-diameter pumping intake pipe was installed in the left abutment of the dam for a future proposed pumping plant (the Helena Valley Pumping Plant).

The powerplant is located at the downstream toe of the dam on the right abutment, and the

three 16,667-kv.-a. turbines are supplied by three 162-inch-diameter penstocks embedded in the dam. The general arrangement of the dam and appurtenant structures is shown in figure 4. The reservoir area extends up the Missouri River Valley for an airline distance of about 24 miles. The reservoir is approximately 411⁄2 miles wide at the widest point, and has a capacity of 2,051,000 acre-feet at elevation 3800.0. Dead storage is 8,000 acre-feet at elevation 3650.0. The remnants of the old Canyon Ferry Dam and Powerplant buildings have been inundated, with Lake Sewell becoming a part of the new reservoir. One county road in the vicinity of the dam was relocated to cross the dam. Area, capacity, and discharge curves for the reservoir, spillway, and outlet works are shown on figure 51, and a more detailed description of the dam and appurtenant features is given in section 33.

3. Physical Geography. The Canyon Ferry unit occupies a portion of the intermountain basin known as Townsend Basin, which lies between the Big Belt and Elkhorn Mountains. These mountains are considered to be subsidiary ranges of the Rocky Mountains. Townsend

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Basin is east of the Continental Divide. About 58 miles above Canyon Ferry Dam the junction of the Jefferson, Madison, and Gallatin Rivers forms the Missouri River. Below the dam the river swings clockwise through a wide flattened arc to a point about 65 miles northeast of Great Falls, Mont., and then flows generally eastward to and through Fort Peck Reservoir and across the Montana-North Dakota State line. The upper portion drains a series of intermountain valleys, but below Great Falls the area along the river is a wide tableland with prairie characteristics, broken by deeply entrenched meandering streams.

Hundreds of thousands of acres of land are used for grazing in the area. Some sections in the upper basin devoted to the raising of grain. crops receive irrigation supplies from mountain streams. Additional thousands of acres of dryland are farmed mainly for the production of small grains. Generally, mountainous areas are well timbered and large sections are included within national forests.

Minerals, both metallic and nonmetallic, are found in considerable quantity throughout the