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<br />The Colorado River, Indian <br />floodplains are moderately <br />residential structures. <br /> <br />Wash, and <br />developed <br /> <br />Horizon Drive Channel <br />with commercial and <br /> <br />2.3 Principal Flood Problems <br /> <br />The principal cause of 100-year flooding along the Colorado River <br />in the study area is rapidly melting snow during May, June, and <br />July. Snowmelt augmented by rain would be required to cause 500- <br />year floods. Snowmelt flooding is characterized by moderate peak <br />flows, large volume, and long duration, and is marked by daily <br />fluctuations in flow. Rainfall on melting snow may hasten the <br />melting process and increase floodflow. General rain floods are <br />caused by prolonged <br />heavy rainfall over large areas and are characterized by high peak <br />flows of moderate duration. Flooding is more severe when <br />antecedent rainfall has resulted in saturated ground conditions or <br />when the ground is frozen and infiltration is minimal. <br /> <br />For the Colorado River, numerous large floods have occurred in the <br />Grand Junction area. The earliest recorded Colorado River flood <br />occurred in June-July 1884. Other floods on that river were <br />recorded in 1917, 1920, 1921, 1935, 1952, 1957, 1983, and 1984. In <br />general, the 1884 flood is considered the most severe known in the <br />Grand Junction area. It resulted from rapid melting of a deep <br />snowpack and concurrent heavy rains. <br /> <br />All major streams in the Grand Junction area were overflowing in <br />May, June, and July 1884, with a maximum peak discharge on the <br />Colorado River at Grand Junction occurring on July 4, with an <br />estimated discharge of 125,000 cubic feet per second (cfs). The <br />1921 flood has an estimated discharge of 81,000 cfs. The recurrence <br />intervals for the 1884 and 1921 floods have been estimated to be <br />300 and 80 years, respectively. Figure 2 shows estimated flood <br />elevations from the Colorado River. The photograph was taken <br />approximately 0.75 mile west of the Colorado River southwest of <br />Grand Junction. Historically, the Colorado River floodflows in the <br />Grand Junction area have inundated streets, lawns, and gardens; <br />deposited sand, silt, and debris; and flooded basements and lower <br />floors in the residential areas in the Riverside Park, Rosevale, <br />and Connecticut Lake sections southwest of the city. The Riverside <br />Park area has had repeated flood danger as the erosion and <br />undermining of protective levees has necessitated extensive flood <br />fighting and levee repair. Several streets in that area as well as <br />Rosevale Road west of the river have been flooded, and traffic has <br />been disrupted. On farmland and ranches south and southwest of the <br />city, crops have been lost and irrigation facilities have been <br />seriously damaged (Reference 5). <br /> <br />The principal cause of flood problems on Indian Wash has been <br />intense localized thunderstorms. These storms often occur over the <br />steeply sloped upper part of the watershed, with little or no <br />precipitation being recorded in Grand Junction. The precipitation <br />intensity rises with increased elevation, as does the velocity of <br /> <br />6 <br />