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I I I I I I I I I I I I I I I I I I I o o W 0') Chapter 1 INTRODUCTION Authority and Purpose The Lower Gunnison Basin Unit (LGBU) was authorized for construction by Public Law 98-569, enacted October 30, 1984. The purpose of the unit is to reduce salt loading to the Colorado River system. The unit area (see general map), which generally encompasses the Bureau of Reclamation's (Reclamation) Uncompahgre Project, annually contributes an estimated 360,000 tons of salt to the Colorado River system. This salt contribution is attributed to seepage losses from unlined canals and laterals as well as deep percolation from irrigation water. Water dissolves salts as it passes through the underlying saline soils and emerges in the Uncompahgre River and eventually reaches the Colorado River. The Feasibility ReportJFinal Environmental Statement (FRlFES) on the unit was filed with the Environmental Protection Agency on February 10, 1984. The plan of development for the entire unit, as presented in the FRlFES, consisted of six components. One of these components would eliminate winter flows throughout the Uncompahgre Project delivery system, while the remaining five would improve the five separate canal and lateral systems which traverse the area east of the Uncompahgre River. Construction of the Winter Water Replacement Program was begun in fiscal year (FY) 1990 and is scheduled to be completed in 1996. Because of the marginal cost effectiveness associated with canal and lateral lining, construction of the last five components was deferred. This Supplement to the Feasibility Report describes a plan and gives a recommendation to pipe the lateral portions and some small canal segments of the five deferred components. Also described are planning activities which have been conducted since the unit was authorized. The report provides a complete description of the proposed plan of development and provides a basis for budget justifications in support of appropriating construction funding. The FR/FES is a primary source for additional and more detailed information and background on the derivation of the salinity control plan authorized for the LGBU. Setting The LGBU, a part of the Colorado River Water Quality Improvement Program, is located in west-central Colorado in the upper Colorado River Basin. The unit area encompasses the Gunnison River drainage area downstream from the Black Canyon of the Gunnison National Monument and upstream from the confluence of the Gunnison and Uncompahgre Rivers near the city of Delta. The unit includes portions of Delta, Gunnison, Montrose, and Ouray Counties.