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<br /> <br />of given recurrence intervals will be decreased. Areas of inundation <br /> <br />and depths and velocities of flow will be correspondingly decreased. <br />Lefthand Creek <br />There are two bridges across Lefthand Creek in the study area, <br />one at the crossing of U.S. Highway 287 and the other (not shown on <br />plate 2) at a farm crossing 2,000 feet above the mouth. The highway <br />bridge offers no constriction to the 50-year or lesser floods; the <br /> <br />farm bridge offers some constriction to the main channel. The main <br /> <br />channel will contain the 25-year flood from the upstream end of the <br />study area to the upstream limit of backwater from St. Vrain Creek. <br />The 50-year flood will exceed the capacity of the main channel at <br />and below the farm bridge (see plate 2). <br />The location of the right boundary of inundation by the 50-year <br />flood below Lefthand Creek will be influenced to some extent by a <br />perched irrigation ditch whose approximate location is shown by the <br />dashed line on plate 2. Near the upstream end of the ditch, its <br />embankments are about 2 feet higher than general valley level, <br />sufficient to deflect the righ~-bank overflow of the 50-year flood <br />on Lefthand Creek. However, toward the downstream end of the <br /> <br />ditch, the embankments approach valley level and will be overtopped, <br /> <br /> <br />thus allowing the flow to assume a general down-valley direction. <br /> <br /> <br />Except in areas very close to the upstream side of the ditch, <br /> <br /> <br />flow on the flood plains of Lefthand Creek will be shallow and <br />average velocities will be low. <br /> <br />28 <br /> <br />