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SUMMARY Authority and Purpose The Lower Gunnison Basin Unit (Winter Water), a part of the Colorado River Water Quality Improvement Program, 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 srea (see Frontispiece Map) annually contributes an estimated 360,000 tons of salt to the Colorado River system. This salt contribution is sttributed to seepage losses from unlined canals and laterals as well as deep perco- lation from irrigation water. Water dissolves salts as it passes through the underlying saline soils and emerges in the llncompahgre River. The Feasibility Report/Finsl Environmental Statement (FR/FES) 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 FR/FES con- sists of six components. One of these components would eliminate and replace winter flows throughout the Uncompahgre Project delivery system, while the remainder would line the five separate high salt content canal and lateral systems which traverse the areas east of the Uncompahgre River. Because of the marginal cost effectiveness associated with canal and lateral lining, construction of the last five components has been deferred until they are considered necessary to meet salinity program goals. This preconstruction report describes the winter water replacement program based on detailed studies which have been conducted since the unit was authorized. The report will serve as the basis for the develop- ment of the program and will also provide the basis for budget justifications in support of appropriating construction funds. Setting The Lower Gunnison Basin Unit 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 Uncompshgre Rivers near the city of Delta. The unit includes portions of Delta, Gun- nison, Montrose, and Ouray Counties. The unit area referred to in this report is that portion which is within the Uncompahgre River Valley and, specifically, the area served by the Uncompahgre Project, a Federal development constructed in the early 1900's. The project provides irrigation water for approximately 86,000 acres by diverting water from the Gunnison River, a tributary of the Colorado River, and the Uncompahgre River, a tributary of the Gunnison River. S-l C017~1