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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />techniques were used to supplement the existing Big Thompson River <br />streamflow record. The results of these efforts are described below. A <br />brief description of the hydrology of the Big Thompson River Basin is first <br />provided as a background. <br /> <br />4.2 HYDROLOGY OF BIG THOMPSON RIVER BASIN <br /> <br />Much of the area drained by the Big Thompson River and its principal <br />tributaries - the North Fork of the Big Thompson River, Buckhorn Creek and <br />Little Thompson River - is in the Rocky Mountains to the west of Loveland. <br />Spring runoff derived from snowmelt in the mountains is the single most <br />important factor affecting the quantity of water available to the City and <br />other owners of direct-flow water rights. During an average year almost <br />three-fourths of the annual total streamflow volume will occur during <br />spring runoff during May through July. <br /> <br />The drainage area of the Big Thompson watershed above the United States <br />Geological Survey (USGS) streamflow gauge at the mouth of the Big Thompson <br />Canyon, is 305 square miles. The total drainage area at its confluence <br />with the South Platte River is 835 square miles. Two major tributaries <br />join the river between the Canyon mouth and confluence with the South <br />Platte. Buckhorn Creek (contributing area of approximately 120 square <br />miles) joins the Big Thompson approximately 3 miles downstream from the <br />Canyon. The Little Thompson River (contributing area of approximately 190 <br />square miles) joins the Big Thompson near Milliken. Of the 16 irrigation <br />ditches or pipelines on the Big Thompson River, only the Evanstown Ditch <br />headgate is downstream from the mouth of the Little Thompson. Most of the <br />remaining drainage area to the Big Thompson River lies along the Front <br />Range. <br /> <br />Streamflows on the Big Thompson vary significantly from one season to the <br />next. The irrigation season typically runs from May through October. <br />During May and June the spring runoff produces high streamflows even though <br />substantial irrigation withdrawals occur. During the July through October <br /> <br />4-3 <br />