Engin firest 11/12/94 39015032 717426 Historic American Engineering Record HAER No. AZ-6 Location: Construction Date: Engineers: Builders: Present Owner: Present Use: Theodore Roosevelt Dam Across the Salt River approximately 60 miles east of Phoenix, at the junction of State Highway 88 and State highway 188, near the confluence of Tonto Creek and the Salt River; Maricopa/Gila Counties, Arizona 1906-1911 U.S. Reclamation Service J.M. O'Rourke and Company, Galveston TX (primary contractor) Water storage and hydroelectric power generation Roosevelt Dam comprises the key structure in one of the first major federally sponsored reclamation project in the West. Authorized as one of the Reclamation Service's first projects in 1903, it continues to store water for agricultural lands, homeowners and industrial concerns in the Phoenix region that are served by the Salt River Project. Used to impound the floodwaters of the Salt River, the 280-foot-high Roosevelt Dam was distinguished as the tallest and last major stone masonry gravity dams at the time of its completion. It was intended to be the flagship of the Reclamation Service, a symbol of government permanency, stability and power. Report Assembled by: Donald C. Jackson, with research assistance by Richard Lynch, in association with FRASERdesign, Loveland, Colorado. June 1992 Introduction HAER No. AZ-6 page 2 Located Mocated approximately 60 miles east of Phoenix in the Mazatzal Mountains, the Theodore Roosevelt Dam is a 280-foot high, 1000-foot-long, curved stone masonry structure, built by the United States Reclamation Service between 1906 and 1911.' It rises 240 feet above the natural river bed, with the remainder of the massive structure extending downward 40 feet to bedrock. The dam is designed as a gravity structure with a maximum thickness of 184 feet at its base and a minimum thickness of 16 feet at the crest. Although Roosevelt Dam is curved in plan along a circular arc with a 400-foot radius, its cross-section is theoretically thick enough to stand safely even if the dam were built in a straight line across the canyon. Spillways located at both ends of the dam have crests 15 feet below the dam's crest. In the 1920s, steel Tainter gates were placed at the mouths of these spillways to increase the reservoir capacity to more than 1.6 million acre-feet [note: an acre foot of water is equal to an area of one acre covered by water one foot deep; this is equivalent to 43,560 cubic feet or about 326,923 gallons]. The addition of the spillway gates represents the only substantive alteration to the Roosevelt Dam proper since its original construction. In contrast, ancillary structures such as the powerhouse, transformer house and spillways have undergone major changes since 1911. When full, the reservoir covers an area of more than 17,000 acres. Impounded water is released through tunnels and allowed to flow directly down the riverbed to Phoenix. As part of the plant used to construct the dam, a hydroelectric generating station was built to provide power. This plant was subsequently adapted as a permanent feature, and large quantities of water being released pass through the turbines of the powerhouse before exiting into the river. Between Roosevelt Dam and Phoenix, three other hydroelectric power dams (Mormon Flat, Horse Mesa and Stewart Mountain) span the Salt River. Built in the 1920s, these facilities also use Roosevelt Dam as a regulating reservoir for their power production. Named for the brackish taste of its waters, the Salt River is one of Arizona's most important streams. In conjunction with its tributary, the Verde River, it drains over 12,000 square miles in the central and eastern parts of the state. The Salt River rises in the headwaters of the Black River at an altitude of over 10,000 feet and flows southwesterly for some 200 miles before joining with the Gila River a short distance west of Phoenix. In its easternmost reaches the Salt drains a rugged mountain region that is HAER No. AZ-6 page 3 covered by substantial snowpack during the winter months. Streams such as White Mountain Creek, Box Canyon Creek and Cherry Creek carry runoff from the melting snowpack and from intermittent rainstorms into the river, causing it to swell on occasion into a raging waterway. Because of variations in precipitation in the Salt River watershed, the natural flow of the river can vary from less than 200 cubic feet of water per second [cfs] in times of drought to more than 100,000 cfs during periods of intense flooding. This extreme variation between low and high water flow marks the Salt River as an excellent stream to regulate using a large storage dam. Floodwaters can be impounded by the reservoir behind the dam, and during dry periods the natural flow of the river can be supplemented by release of some of the accumulated storage waters. Ideally, a damsite along a major river should be at an elevation sufficiently low to capture most of the runoff within the watershed, yet high enough to prevent excessive evaporation from the reservoir and provide service to large tracts of irrigable lands at lower elevations through gravity canals. Along the Salt River an ideal site exists at an elevation of about 1,900 feet above sea level. Called The Crossing by early valley settlers because of its use as a ford by Indians, farmers and ranchers, this site is situated in a narrow gorge a short distance below the confluence of the Salt River and a tributary from the north known as Tonto Creek. Below this site the Salt River courses through a narrow canyon for several miles; above it is a relatively large, flat expanse of land known as the Tonto Basin. Originally known as the Tonto site, this natural dam location attracted the interest of farmers in Arizona's central valley in the late 19th century as they sought the means to increase their supply of water. It eventually became the site of Roosevelt Dam. (See figure 1.) Irrigated agriculture by Anglo settlers in the Salt River Valley had started shortly after the Civil War and initially used the remains of ancient canals and ditches built by the longvanished Hohokam Indians. Early irrigation depended upon low diversion dams to direct the flow of the Salt River into canals, which then carried water to the fertile lands surrounding the confluence of the Salt and Gila Rivers. The application of this water on otherwise arid land allowed for the nourishment of crops and, in a phrase popular among late 19th century irrigation enthusiasts, it "let the desert bloom." By the 1890s Phoenix residents were actively seeking ways to increase their water supply by impounding floodwaters behind a large storage dam. With the formation of the U.S. Reclamation Service [later renamed the Bureau of Reclamation] in 1902, Phoenix found a source of federal funding for building such a structure across the upper Salt River. Although formal ownership of the resulting Roosevelt Dam remains in federal hands, its operation has been controlled since 1917 by the locally based Salt River Valley Water Users' Association. |