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<br /> <br />5. Streamflow Records. Streamflow data for this study comprise records <br />for U.S. Geological Survey gaging stations in the Gunnison River basin. <br />Major gages used in the study, together with maximum floods of record, are <br />listed in Table 2 (on page 5). Additional information on these gages and <br />information on other gages in the basin is contained in Appendix A. <br /> <br />6. Nature of Flood Problems. As noted, most of the annual precipitation <br />in the higher elevations of the Gunnison River basin occurs as snow, and a <br />deep snowpack accumulates. General rainstorms covering large areas for ex- <br />tended periods and convective-type cloudburst storms of small areal extent <br />can occur anytime fronl late May through Septetlber. <br /> <br />Rapid melting of D~untain snowpacks in spring and early summer is the <br />major cause of flooding along the North Fork Gunnison, Gunnison, and upper <br />Uncompahgre Rivers. Flooding is usually more severe when snowmelt is aug- <br />mented by rain. Annual peak flows on the lower Uncompahgre River are <br />caused by runoff from either sno~~elt or, less frequently, rainfall. The <br />incidence of peaks from rainfall runoff is attributed to several factors: <br /> <br />a. Snowpack covers a proportionately smaller area in the Uncompahgre <br />River drainage than in the North Fork Gunnison and Gunnison River drainages <br />because the mean elevation of the basin is lower. <br /> <br />b. Below Colona, snowmelt runoff attenuates due to channel storage <br />and numerous diversions. <br /> <br />c. Rainstorms cover proportionately larger areas in the Uncompahgre <br />River drainage than in the North Fork Gunnison and Gunnison River drainages. <br /> <br />7. Flood history. The largest flood known to have occurred in the Gunnison <br />River basin was in June 1884. It resulted from rapid melting of a deep <br />snowpack and concurrent heavy rains. A similar flood probably occurred on <br />the Uncompahgre and North Fork Gunnison Rivers. <br /> <br />The largest recorded discharge on the Gunnison River was 35,700 cfs, <br />which was measured at the "near Grand Junction" gage in May 1920. A maxi- <br />mum mean daily flow of 4,080 cfs at the Colona gage in June 1921 was the <br />largest flow of record on the Uncompahgre River. The peak flow for this <br />event is not available. 11axliaum floods at selected locations in the study <br />area are shown in Table 2. <br /> <br />In general, high flows along the North Fork Gunnison, Gunnison, and <br />Uncompahgre Rivers are contained in-channel, and extensive overbank flood- <br />ing is infrequent. <br /> <br />4 <br />