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<br />2 <br /> <br /> <br />. <br /> <br />contours shown on the base map represent th? topography of the area <br />as it was in 1950-51, within the limits of acc~racy specified by <br />the Geological Survey for topographic mapping at the published <br />scale and contour interval. One of the specifications is, in part: <br />"Vertical accuracy**** shall be such that not more than 10 percent <br />of the elevations tested shall be in error more than one-half the <br />contour interval." <br /> <br />. <br /> <br />Previous work <br /> <br />Studies that the author made for two previous reports (1960 <br />and 1961) concerning Boulder Creek, a tributary of St. Vrain Creek, <br />were useful in the preparation of this report. The 1960 report <br />was published in condensed form as Hydrologic Atlas 41, Floods at <br />Boulder, Colorado. <br /> <br />Scope <br /> <br /> <br />The area considered in this study includes the valley of St. <br /> <br /> <br />Vrain Creek between Hover Road near the west city limits of Longmont <br /> <br /> <br />and County Highway 3 about three miles downstream, and the valley <br /> <br /> <br />of Lefthand Creek downstream from U.S. Highway 287 (see plate 2). <br /> <br /> <br />The hydrologic and hydraulic factors presented are: magnitude and <br /> <br /> <br />frequency of floods, flood history of St. Vrain and Lefthand Creeks, <br /> <br /> <br />estimated boundaries of areas inundated by floods of selected <br /> <br /> <br />recurrence intervals, estimated flood profiles along the thalweg <br /> <br /> <br />of St. Vrain Creek, estimated depths of flow and water-surface ele- <br /> <br /> <br />vations at selected points, and a general discussion of the possible <br /> <br />, <br /> <br />,j, <br />