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<br />vegetation encroaching upon channel banks. The flood plains are <br />largely undeveloped for both streams, and the soils are of the <br />Argiborolls-Rock Outcrop and Cyroboralfs-Rock Outcrop Associations <br />(Reference 4). <br /> <br />The Cache La Poudre River f~ows in a southeasterly direction <br />through Larimer County. Development along the Cache La Poudre <br />River flood plain is minor at the present time with only a few <br />residential and commercial structures. The channel in the studied <br />section is approximately 160 feet wide and 7 feet deep, wi,th the <br />gradient varying from 28 feet per mile in the upper parts of the <br />study area and 16 feet per mile in the lower parts. The soils <br />within the Cache La Poudre River study area consist of the <br />Fluvaquents-Fluvents Association (Reference 4). <br /> <br />81ack Canyon Creek was studied from its confluence with 8ig <br />Thompson River upstream approximately 1.4 miles to McGregor Avenue. <br />The upstream 0.4 mile of this study reach is in Larimer County, <br /> <br />. <br /> <br />2.3 Principal Flood Problems <br /> <br />Major floods on the streams are caused by intense rainfall from <br />localized thunderstorms over the basins. ,The floods caused by such <br />events will characteristically have high peak discharges of short <br />duration for all of the mountain-area streams. Floods on the <br />streams in the plains areas are normally of a longer duration with <br />less velocity and considerable channel storage. Flooding can also <br />occur as a result of rapid spring snowmelt. These floods <br />characteristically are of longer duration. <br /> <br />Significant floods have occurred on the streams within the county <br />in past years. The most significant flood within the county <br />occurred on the Big Thompson River, July 31 to August 1, 1976. <br />This flood was one of the worst natural disasters in the history of <br />Colorado, Intense precipitation over an approximate 60-square-mile <br />area between Lake Estes and Drake, with rainfall depths up to 12 <br />inches, generated a flood discharge of approximately 31,200 cubic <br />feet per second (cfs) at the mouth of the canyon. This flood is <br />known to have taken 139 lives. Property damage was estimated at <br />$16.5 million, while hundreds of people were left homeless. Over <br />200 residential structures were damaged or destroyed by the flood, <br />while nearly 1,200 land parcels were adversely affected <br />(Reference 5). Floods on the Big Thompson River caused damage in <br />1864 and 1894, but no discharge or damage estimates were recorded. <br />Floods also occurred on the Big Thompson River in 1919, 1923, 1945, <br />and 1949 with discharges of 8,000, 7,000, 7,600, and 7,750 cfs, <br />respectively, The 1976 flood is depicted in Figures 2 through 6. <br /> <br />" <br /> <br />, <br /> <br />Notable floods on the Cache La Poudre River in the study area <br />occurred in 1844, 1864, 1884, 1891, 1904, 1923, and 1930. There <br />were apparently three large floods of comparable size in 1864, <br />1891, and 1904. All of these floods peaked near 21,000 cfs. The <br />1904 flood was probably the worst flood in terms of dollar damage <br /> <br />6 <br />