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<br />2.3 Principal Flood Problems <br /> <br />The principal' cause of 100-year flooding along Colorado River <br />in the study area is rapidly melting snow during May, June, and <br />July. Snowmelt augmented by rain would cause 500-year floods. <br />Snowmelt flooding is characterized by moderate peak flows, large <br />volume, and long duration, and is marked by daily fluctuations <br />in flow. Rainfall on melting snow may hasten the melting process <br />and increase floodflow. General rain floods are caused by prolonged <br />heavy rainfall over large areas and are characterized by high <br />peak flows of moderate duration. Flooding is more severe when <br />antecedent rainfall has resulted in saturated ground conditions <br />or when the ground is frozen and infiltration is minimal. <br /> <br />For 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 recorded <br />in 1917, 1920, 1921, 1935, 1952, and 1957. In general, the 1884 <br />flood is considered the most severe known in the Grand Junction <br />area. It resulted from rapid melting of a deep snowpack and concur- <br />rent heavy rains. <br /> <br />All major streams in the Grand Junction area were overflowing <br />in May, June, and July 1884, with a maximum peak discharge on <br />Colorado River at Grand Junction occurring on July 4 with an esti- <br />mated discharge of 125,000 cubic feet per second (cfs). The 1921 <br />flood has an estimated discharge of 81,000 cfs. Based on the <br />results of this study, the recurrence intervals for the 1884 and <br />1921 floods are 300 and 80 years, respectively. <br /> <br />Figure 2 shows estimated flood elevations from Colorado River. <br />The estimated flood heights were developed by the U.S. Army Corps <br />of Engineers for the Mesa County, Colorado, Flood Insurance Study <br />(Reference 1). The photograph was taken approximately 0.75 mile <br />west of Colorado River southwest of Grand Junction. <br /> <br />. <br /> <br />Historically, Colorado River floodflows in the Grand Junction <br />area have inundated streets, lawns, and gardens; deposited sand, <br />sil t, and debr is; and flooded basements and lower floors in the <br />residential areas in the Riverside Park, Rosevale, and Connecticut <br />Lakes sections southwest of the city. The Riverside Park area <br />has had repeated flood danger as the erosion and undermining of <br />protective levees has necessitated extensive flood fighting and <br />levee repair. Several streets in that area as well as Rosevale <br />Road west of the river have been flooded, and traffic has been <br />disrupted. On farmland and ranches south and southwest of the <br />city, growing crops have been lost, and irrigation facilities <br />have been seriously damaged (Reference 1) . <br /> <br />Historically, the principal cause of flood problems on Indian <br />Wash has been intense localized thunderstorms. These storms often <br />occur over the steeply sloped upper part of the watershed, with <br />little or no precipitation being recorded in Grand Junction. <br /> <br />6 <br />