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. (l, ::'y"; )l ..~ '5 n "J C~"".I.~ . . . . . . . . . . PART!. SUMMARY The La Plata River drains a 440 square mile area near the southwestern corner of Colorado and discharges into the San Juan River at a point about 19 miles south of the Colorado-New Mexico state line. The general location of the La Plata River basin is shown on Figure 1. The river flows from north to south, with its headwaters originating in the La Plata Mountains. Elevations vary from 13,000 feet in the La Plata Mountains to 6,000 feet at the CoIorado- New Mexico state line. Annual precipitation varies from over 40 inches in the mountains to about 10 inches near the state line. The headwaters of the La Plata River provide a water supply for irrigation of up to 15,300 acres in the area generally south of Highway 160, with irrigation extending south to the state line. May and June are months of maximum river flow, resulting from snowmelt high in the mountains. During the remainder of the year, the river has a much lower flow with minimums occUlTing from August through January. The natural flow of Cherry Creek, a major tributary of the La Plata River, peaks earlier in the spring and nearly dries up by July. Flow in other tributaries is typically short-term response to snowmelt and infre- quent rainstorm runoff. Long-term gaging station records are available for two locations along the La Plata River. The recorded flow of the La Plata River at Hesperus is affected by three diversions for ilTigation upstream of the gage and the virgin flow is therefore equal to the historical gaged flow plus the amounts diverted at the three irrigation diversions. The virgin flow of the La Plata River at the Colorado-New Mexico state line was considered to be equal to the virgin flow at Hesperus plus inflow contri- butions to the river downstream of Hesperus, less river loss. Inflows consist of two components: (1) short-term runoff that occurs in response to precipitation from thunderstorms during summer and fall and (2) other river gains and base flow -1- ',~ ".'f: