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I, I ~ I .; .: :~ ~ - " J I / f .I " 1. INTRODUCTION 1.01 BACKGROUND Trinidad Dam and Lake are located on the upper Purgatoire River, a right bank tributary of the Arkansas River, about 2.75 miles upstream and southwest of the city of Trinidad, Las Animas County, south central Colorado (Figure 1). The U.S. Army Corps of Engineers, Albuquerque District (Corps) received Congressional authorization to construct the dam in July of 1958 (Public Law 85-5(0), and construction was completed in 1976. The multi-purpose reservoir is authorized for flood control, irrigation, sediment retention, and recreation. The initial authorized allocation of storage is as follows: 51,000 acre-feet - Flood Control 20,000 acre-feet - Irrigation 39,000 acre-feet - Sediment 4,500 acre-feet - Recreation The sediment space allocation is based on a 75-year project design. Of the initial 39,000 acre-feet authorized for sediment, 37,325 acre-feet remain as of the 1986 sediment survey. The unused sediment space is available for joint-use storage of irrigation water. .. - Trinidad Lake is a water resource project jointly developed by the Corps and the Bureau of Reclamation (Bureau). The Corps owns and operates the reservoir with major responsibilities for flood control. The Bureau has the federal responsibility to administer a contract with the Purgatoire River Water Conservancy District (PRWCD), which manages the irrigation capacity -~f the reservoir. Water rights are administered by the state of Colorado. A set of Operating Principles, and five conditions proposed by the state of Kansas, were developed for the operation of Trinidad Lake. These Operating Principles, which address all types of storage in the project, were accepted by the PRWCD and approved by the Arkansas River Compact Administration. The Trinidad Lake Water Control Manual contains the detailed Water Control Plan. The manual serves as the guide to 'personnel involved in the flood control regulation of the project. The recreation pool referred to in the'Water Control Manual is the same as the "permanent fishery pool" referred to in the Operating Principles. 1.02 PURPOSE AND NEED Approximately 11,467 acre-feet of storage has been identified below the top of the irrigation pool that is not specifically allocated. This is in excess of the Congressionally authorized amounts for the project. The scope of the federal action covered by fuis Environmental Assessment (EA) is to address the disposition of the 11,467 acre-feet to provide the most effective and beneficial use of this excess storage within the lake through a revision of the Trinidad Lake Water Control Manual. 1