Hydrology of the Reservoir and Surround Area. The Mancos Valley, before construction of the Project, was dependent for its water supply on the natural flow of the east, middle and west branches of the Mancos River. An average of 12% of the flow of the river occurs during, July, August and September of most years, and it is only sufficient to satisfy the first 4 to 8 adjudicated water rights after July. The Project was authorized to relieve those hydrologic shortages and assure an adequate supply of water for the late season irrigation use. (p.1, Final Report, 1952) Average annual precipitation in the area is 15.9 inches with winter snowstorms dropping as much as 4 feet during a major storm. The ground remains snow covered through March with the beginning of spring thaw and runoff. The West Fork of the Mancos River is a primary waterway in the area. It gathers runoff from numerous streams emanating from higher elevations in the south and west sectors of the La Plata Mountains. The river is characteristic of many western watersheds with high spring flows from melting snowpack and low summer, fall, and winter flows. Such systems have been capable of sustaining economically viable agricultural projects. The lower reaches of this stream has been extensively develop for irrigation and other uses for the last 100 years. The Project diverts water from the West Fork through a concrete diversion structure located in SE 1/4 Sec. 25, Township 37 North, Range 13 West, New Mexico Principal Meridian, Montezuma County Colorado. The Inlet Canal runs southwesterly for 2.6 miles to Xxxxxxx Gulch Reservoir (surface area of 216 ac). Water then exits through outlet works on the east side of the Xxxxxxx Gulch Dam (contains hydroelectric facility) and returns flows south and east down the Outlet Canal for 2.2 miles to the West Fork of the Mancos River The fluctuation of the surface elevation of the Xxxxxxx Gulch Reservoir varies greatly from year to year depending on water demand and runoff. The reservoir is typically filled in the spring with drawdown occurring gradually through the summer. (MSP, 1994)
