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<br />001963 <br /> <br />3.2 Hydrology, Stream Flows and Water Quality <br /> <br />3.2.1 Hydrology of the Reservoir and Surround Area <br /> <br />The Mancos Valley, before construction of the Project, was dependent for its water supply on the <br />natural flow of the east, middle and west branches of the Mancos River. An average of 12% of <br />the flow of the river occurs during, July, August and September of most years, and it is only <br />sufficient to satisfy the first 4 to 8 adjudicated water rights after July. The Project was <br />authorized to relieve those hydrologic shortages and assure an adequ,atesupply of water for the <br />late season irrigation use. (p.1, Final Report, 1952) <br /> <br />Average annual precipitation in the area is 15.9 inches with winter sn6W~~9nns dropping as <br />much as 4 feet during a major storm. The ground remai~~Il.ow,~overed'tlU'9ugh March with the <br />beginning of spring thaw and runoff. The West Forkoftne Mancos River is~prjmary waterway <br />in the area. It gathers runoff from numerous streamsefTIanatingfrom higher ekvations.jn the <br />south and west sectors of the La Plata Mountains;;Th~cB:ver is cltaracteristic of many western <br />watersheds with high spring flows from melting snowpack.~cl\'low'summer, fall, and winter <br />flows. Such systems have been capable of sustaining econoIijically viable agricultural projects. <br />The lower reaches of this stream has been extensively develoI#orirrigation and other uses for <br />the last 100 years. <br /> <br />The Project diverts water from the West Forkthrough/a'cpn<;rete diversion structure located in <br />SE 1/4 Sec. 25, Township 37 North, Range 13We~t;NewcM'~:lticb Principal Meridian, <br />Montezuma County Colorado.'l'~e Inlet Canah}.lns southwesterly for 2.6 miles to Jackson <br />Gulch Reservoir (surf(i{;earea 0['2,;1(> ac). Waterthen exits through outlet works on the east side <br />of the Jackson Gulcl1])~m (contaiI:f$hydroelectricf(l(;i1.ity) and returns flows south and east <br />down the Outlet Canal for.2~2 miles;to.the West Fork'of the Mancos River <br /> <br />The fluctuatiotf6fthesurface elevation of the Jackson Gulch Reservoir varies greatly from year <br />to year d~pending onWat,er.demandand runoff. The reservoir is typically filled in the spring <br />with <hawg.own occurringgrliduallytl#()l,lgh the summer. (MSP, 1994) <br /> <br />3.2.2 Irrig~tt()nReturn FloW!!! <br /> <br />All of the projecf1:alI<!s receiving project and private water lie within the San Juan River <br />Drainage. Return flQ~~,gci1n these irrigated lands flow into the San Juan drainage. In the San <br />Juan drainage, the subj~cf465 af (Colyer, Spencer, and Robbins) has historically been used and <br />reused for irrigation. Estimates for historical depletion of that water range as high as 85%. <br />Specifically, return flows from these lands are intercepted by the West Fork and Middle Fork of <br />the Mancos River and Chicken Creek which in turn flow into the Mancos River and finally the to <br />San Juan River. <br /> <br />.; <br />V <br /> <br />Reclamation does not expect any new additional depletions to occur as a result of these proposed <br />water contracts. <br /> <br />12 <br />