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<br /> <br />extended historical period. A generalized skew coefficient of -0.30 was <br />used to weight the station skew coefficient, resulting in an adopted skew <br />coefficient of -0.40. The peak flow-frequency curve developed from this <br />analysis is shown on Chart 5. <br /> <br />b. Gunnison River near Grand Junction. Records of unregulated flow <br />for the "near Grand Junction" gage are available for 1897-99, 1902-06, and <br />1917-1976. The 1884 flood was estimated to have a peak of 45,000 cfs based <br />on comparisons with observed and estimated flows at other gages in the <br />Colorado River basin. The estimated peak was weighted with the recorded <br />data set using the extended historical period. A generalized skew coeffi- <br />cient of -0.30 was used to weight the station skew coefficient, which re- <br />sulted in an adopted skew coefficient of -0.45. The resultant peak flow- <br />frequency curve is shown on Chart 6. <br /> <br />c. Gunnison River below Gunnison Tunnel. :'leasurelOents of unregulated <br />flow on the Gunnison River below Gunnison Tunnel were Dade for 1906-1965. <br />Peak flows for 1900-1903 were estimated from a peak flow correlation with <br />the gage upstream at Iola based on the common years of records, 1938-1950. <br />The 1884 flood was estin~ted to have a peak flow of 27,000 cfs. This value <br />was based on a comparison with other major floods, which indicated that the <br />peak flow at the "below Gunnison Tunnel" gage is ahout 60% of the peak flow <br />at the "near Grand Junction" gage. The estimate was then weighted with the <br />recorded data, using the extended historical period. However, this analysis <br />produced a frequency curve with a much higher negative skew (-.90) than <br />seemed reasonable. If the frequency curve were to have a skew as low as <br />the computed station skew coefficient, the upper portion of the curve <br />would "flatten" and the l,OOO-year flood would have a peak of about 21,000 <br />cfs. This peak is only slightly more than the peak of 19,000 cfs for the <br />1921 flood and considerably less than the estimate for the 1884 flood. The <br />frequency curve was therefore adjusted to have a skew of -0.50, which is <br />more consistent with skew coefficients for other gages in the basin. The <br />adopted curve produces frequencies of observed peaks that are consistent <br />with the frequencies derived for the same events at other gages in the <br />basin. The peak flow-frequency curve adopted for use is shown on Chart 7. <br /> <br />d. Uncompahgre River at Colona and at Delta. Streamflow records are <br />available for the Uncompahgre River at Colona for 1903-1905 and 1921- <br />present and at Delta for 1902-1931 and 1938-present. As noted, annual <br />peaks on the lower reaches of the Uncompahgre River, particularly at Delta, <br />are sometimes caused by rain. Therefore, annual peaks for rain and snow- <br />melt runoff were analyzed independently, and frequency curves were developed <br />for each. The historical record of type of event versus number of annual <br />peak flows is shown in the following tabulation. <br /> <br />8 <br />