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~ ..... ~.. (", ~c~.J INTRODUCTION Pueblo Dam and Reservoir, located on the Arkansas River in Pueblo County, are 6 miles west of the city of Pueblo in south-central Colorado. Pueblo Reservoir, formed by Pueblo Dam, is one of the major storage features of the Fryingpan-Arkansas Project. The Fryingpan- Arkansas Project is a multipurpose, transmountain development which diverts water from the Fryingpan River and tributaries on the western slope of the Rocky Mountains to the Arkansas River on the eastern slope. Additional reservoirs that form the project include Ruedi Reservoir on the western slope and Turquoise Lake, Mt. Elbert Forebay Reservoir, and Twin Lakes Reservoir on the eastern slope. Pueblo Reservoir is the largest and the terminal storage feature of the Fryingpan-Arkansas project (fig. 1). Construction of the Fryingpan-Arkansas Project was authorized by Public Law 87-590 (76 Stat. 389) and signed by the President on August 16, 1962. Construction of Pueblo Dam began in 1970 and was completed in August 1975. The left embankment was modified in 1980 by the addition of a downstream stability berm to provide protection against potentially weak foundation soils. Initial water storage began on January 9, 1974. Maximum elevation was attained in water year 1986 at reservoir elevation 4887.1 feet. Pueblo Dam is a composite concrete and earthfill structure, about 10,200 feet long at crest elevation 4925.0. The concrete section has a structural height of 250 feet and a hydraulic * height of 191 feet. The earthf1l1 portions consist of the left and right abutment embankments, totaling 8,450 feet in length. The concrete dam consists of 23 massive-head buttresses which total 1,750 feet in length, and has a 550-foot overflow spillway section and a .l,200-foot nonoverflow section (fig. 2). The uncontrolled overflow spillway has a crest elevation of 4898.7 feet and is located in the overflow section of the buttresses. The spillway consists of a concrete ogee crest, training walls, flip bucket, stilling basin, and an outlet channel. The spillway design flow is 191,500 ft3/s (cubic feet per second) at reservoir elevation 4919.0. Five separate outlets operate at Pueblo Dam. The Bessemer Ditch, located in the right embankment, is controlled by four high-pressure gates with a maximum discharge of 393 ft3/s. The river outlet, located in the buttress over the streambed, is controlled by two gates with a maximum discharge of 1,120 ft3/s. The spillway outlets, located under the spillway, consist of three gated conduits with a maximum discharge of 8,190 ft3/s. The fish hatchery outlet, located in the buttress, is controlled by multilevel gated intakes for controlling water quality to the downstream fish hatchery. The maximum discharge rate is 30 ft3/s. The south outlet, located in a buttress, has gated intakes and supplies a maximum discharge of 345 ft3/s of municipal and industrial water (fig. 3). Pueblo Reservoir has an approximate length of 11.4 miles and an average width of 0.8 miles at reservoir pool elevation 4900.0. The average width is determined by dividing the surface area by the reservoir length at elevation 4900.0. The total Arkansas River drainage area above the dam is 4,669 square miles, of which an estimated 4,170 square miles contributes . The definition of terms such as "structural height," "hydraulic height," etc., may be found in manuals such as Reclamation's DeSign of Small Dams and Guide for Preparation of Standing Operating Procedures for Dams and Reservoirs, or ASCE's Nomenclature for Hydraulics.