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<br />3 <br /> <br /> <br />. <br /> <br />. <br /> <br />effects of alterations to the valley of St. Vrain Creek and its <br />constrictions. <br /> <br />Elevations are referred to mean sea level datum. <br /> <br />The data and computations supporting the results given are in <br /> <br />the files of the U. S. Geological Survey, Denver, Colorado. <br />ST. VRAIN CREEK AND TRIBUTARIES <br />Above Study Area <br />St. Vrain Creek is formed by the confluence of North and South <br />St. Vrain Creeks at Lyons, Colorado, about 9 miles upstream from <br />Longmont. The headwaters of the creeks are at the Continental Divide <br />at elevations that reach a maximum of 13,500 feet. The elevation at <br />Longmont is about 5,000 feet. Features of the general area are <br /> <br />shown on plate 1. Above Lyons, the streams are confined to relatively <br /> <br />narrow canyons and have precipitous slopes. Below Lyons, St. Vrain <br /> <br />Creek enters the Plains region and is characteri~ed by moderate slopes, <br /> <br />a well-defined main channel, and wide, flat, overflow plains that <br />are under cultivation or in pasture. There are many diversions to <br /> <br />off-channel reservoirs and to irrigation ditches below Lyons. The <br /> <br />diversion structures are low and provide little main channel storage. <br /> <br />Storage on tributaries and diversions above Lyons are minor and have <br /> <br /> <br />negligible effect on flood flows. <br /> <br /> <br />The drainage area of St. Vrain Creek at Lyons is 212 square <br /> <br /> <br />miles; at U.S. Highway 287 in Longmont, 295 square miles. <br /> <br />~ <br />