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<br />I <br />I <br />, <br />I <br /> <br />, <br />, <br />i <br />I <br />I. <br />I <br />! <br /> <br />MACNI~JDE AND FREQUENCY OF FLOODS <br /> <br />Based on Eyewitness end Historic Information <br /> <br />The South Platte, Cache la Poudre, Big Thompson', and lesser <br /> <br />rivers have all had several major floods since settlement began <br /> <br />(Table 1). Host of these floo.ds were not gauged. They have been <br /> <br />described as being "from bluff to bluff," "across the bottom <br /> <br />lands,"or "a mile and a half wide." The number of major floods <br /> <br />were counted for each river, and divided by the years of record <br /> <br />yielding values of 1/14, 1/16, 1/17 for the Cache la Poudre, <br /> <br />South Platte and Big Thompson Rivers respectively. This does not <br /> <br />imply that the floods were of equal size or probability. It can <br /> <br />only be said that based on previous experience; these rivers can <br /> <br />be expected to flood several times in a lifetime. <br /> <br />Eyewitness and historic accounts make it possible to say <br /> <br />I <br />I <br />i <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />something about how often floods of the magnitude mapped on the <br /> <br />creeks have occurred. The flood mapped on LoneTree Creek is <br /> <br />approximately the size of the three largest floods'in sixty years. <br /> <br />The flood mapped on Owl Cre,ek is the largest flood in at least <br /> <br />thirty years. The area that has been mapped on Boxelder Creek <br /> <br />is significantly smaller than an area that was flooded at. least <br /> <br />once in the last 120 years. It is assumed that the flood histories <br /> <br />of other small creeks are similar. From all of the foregoing, one <br /> <br />would expect the areas shO\m on the maps to flood more than Once <br /> <br />in a lifetime, but probably not as often as once every ten years. ' <br />