Rehabilitation and betterment of the project works was authorized by the act of October 7, 1949 (63 Stat. 724) as amended. The general nature and purpose of the rehabilitation and betterment program was to reduce operation and maintenance costs, improve the irrigation facilities, increase operating efficiency, and conserve available water supplies.

The construction of the dam was started in 1903 and was completed in 1911. This structure was the first multipurpose project built by the Bureau of Reclamation. Its unique cyclopean-masonry thick arch design has resulted in its being listed in the National Registry of Historic Places. In 1971, the American Society of Civil Engineers recognized the site with the National Historic Engineering Landmark Award.

In 1986, modification began on the existing dam to mitigate safety of dams issues associated with the probable maximum flood. The major features included raising the existing dam by 76 feet, constructing a top-seal radial-gated spillway on each abutment, and constructing a lake-tap river outlet works/power penstock through the left abutment. The dam modification was designed as a single-curvature, uniform thickness mass concrete arch dam. The dam was raised  to El. 2218. The upstream face of the modified dam is a vertical projection of the upstream face of the existing dam. The alignment of the modification, therefore, is the same as that of the existing dam. The shape of the existing and modified dam includes a central arch portion with tangent sections on both sides leading to thrust blocks on the abutments. The existing spillway structures will be removed from both abutments and replaced by the thrust blocks. The thrust blocks will provide abutments above elevation 2100.0 and will contain the spillways. In section, the modification will have a vertical upstream face, an 18-foot crest thickness, and a downstream slope of 0.45 to 1.00.

The two top-seated radial gated spillways in combination with significant reservoir storage is required to regulate flood releases. The existing topography and existing conditions as well as economic considerations led to the design of two gated spillway structures, one on each abutment, in the same general vicinity as the existing spillways. To dissipate the energy of the spillways which flip the discharge into the downstream canyon, a plunge pool was constructed. Spillway layouts were developed to minimize the variables which influence the size and depth of the plunge pool. The right spillway is straight in plan and the left spillway is curved in plan with an associated superelevation. The spillway alignments cause the discharge jets to impinge at or above the tailwater within the plunge pool. The result of the impingement is the dissipation of some of the energy of the discharge jets and a a resultant downstream component to the discharge jets.

The river outlet works was designed to be used for reservoir evacuation, flood releases, and normal releases. The reservoir evacuation criteria governed the sizing of the system. This feature was constructed under approximately 130 feet of water. It consists of a trashrack structure, a 16-foot diameter shaft, a 16-foot diameter steel lined tunnel, a 22-foot diameter wet-well gate shaft which houses a fixed-wheel gate used for emergency closure, and a tunnel. The tunnel bifurcates into two 10-foot, 9-inch steel lined tunnels, where one will serve the powerplant and the other the river outlet works.

 | Operator:  Salt River Project Agricultural Improvement & Power District,
                        PO Box 52025, Phoenix, AZ 85072-2025, (602) 236-5900