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,I' or10115," "1' ,'.,..,' }' ,- i ," WATER SUPPLY AND DEPLETIONS OF THE SAN JUAN RIVER IN COlDRAOO I. Introduction The purpose of this report is to evaluate the effect of the man-made depletions, both present and future, on the water supply of the San Juan River and its tributaries in Colorado. The San Juan River, which joins the Colorado River in southern Utah, is the second largest tributary of the Colorado River, furnishing 20 percent of the flow of the Colorado River at Lee's Ferry. The source of the water supply of the San Juan River is primarily in the mountains of southwestern Colorado. The tributary areas of the San Juan can be classified according to sub-basins. The easternmost of these sub-basins is the Upper San Juan Basin which includes such tributaries as the Navajo River, the Rio Blanoo, the East Fork of the San Juan River, the West Fork of the San Juan, and numerous smaller streams. The sub- basins to the west are named by the main streams that traverse the basins. These include the Piedra River Basin, the Los Pinos River Basin, the Florida and Animas Rivers Basins, the La Plata River Basin, the Mancos River Basin, and the McElmo Creek drain- age area. The McElmo Creek watershed drains into Utah before joining the San Juan River. Future developments of the water supply of the San Juan River will deplete the flow at the state line. This report attempts to evaluate these depletions and their effect on the historic flow that would occure during a dry period, su~h as 1934 and through 1954. - The Dolores Project will increase the flow of the San Juan " River because the diversion will be from the Dolores River to lands . ...L. ; ,._~.