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, National Water Summary 198B-89-Floods and Droughts: COLORADO 211 ured to the June 1965 flood on the South Platte River and total dam- age was $508 million, of which about 75 percent was in the Denver metropolitan area. Peak discharges at several gaging stations had recurrence intervals that exceeded 100 years. The peak discharge on the South Platte River at Denver was 40,300 ftJ/s (cubic feet per second), which is 1.8 times greater than the next largest discharge of record since 1889. Matthai (1969, p, B-36) reported a peak dis- charge on East Plum Creek near Castle Rock of 126,000 ff'ls from a contributing drainage area of 108 mP, During June 17-19, 1965, moderate to severe flooding occurred in the Arkansas River basin (fig. 3). The flooding was caused by extreme rainfall on June 16 and 17 following 2 days of moderate rainfall; snowmelt was a minor contributor. Widespread rainfall in May and early June created moist antecedent conditions in most areas affected by the flood. The peak discharge of the Purgatoire River at Ninemile Dam, near Higbee (fig. 3, site 3) on June 18 became the peak discharge of record for that st,ation and has remained the peak discharge of record to the present (1988). Peak discharges during the June 1965 floods in the Arkansas River basin in Colorado, Kansas, and New Mexico were greater than those previously recorded at 48 of 136 other gaging stations as well, and many peak discharges had recurrence intervals that exceeded 100 years. Sixteen lives were lost due to the flooding, and property damage was about $60 million (Snipes and others, 1974, p. 0-1). The flood of July 3 I-August I, 1976, on the Big Thompson and Cache la Poudre Rivers. resulted in at least 144 deaths and total damage of about $39 million (McCain and others, 1979, p. 70, 71). Some of the peak discharges on the Big Thompson River were ex- tremely rare; the largest was about four times that having a IOO-year recurrence interval. Other peak discharges, especially on the Cache la Poudre River, were not as significant. This flood was produced by 6-12 inches of rainfall from a SlOIm centered over the downstream pan of the Big Thompson River basin during the evening of July 31, 1976. Many of the lives lost were campers who had set their camps near the river. The flood of July 15, 1982, on the Roaring and Fall Rivers resulted from failure of a 26-foot-high earthen dam that fonned a small irrigation-storage reservoir at an altitude of about 11,000 feet. According to Jarrett and Costa (1986, p. 6), the most likely cause of ~ dam failure was erosion around the outlet pipe, which eventually weakened the dam, causing a breach. The dam failure released 674 acre-feet of water into the Roaring River, which then reached an es- timated peak discharge of 18,000 fels within about 10 minutes. The floodwaters moved downstream into the Fall River and tipped over a concrete gravity dam at a smaller reservoir. Downstream from that reservoir, the peak discharge was estimated to be 16,000 fels or larger. The floodwaters then passed through the resort town of Estes Park and into Lake Estes (altitude about 7,500 feet), where the total volume was contained. The floodwaters moved 12.5 miles from the first reservoir into Lake Estes in about 3 hours and 40 minutes. During that time, the flood caused three deaths and $31 million in damage (Jarrett and Costa, 1986, p. 2). The floods of June and July 1983 were the direct result of snowmelt combined with minor rainfall runoff at lower altitudes in the drainage basins of the upper Colorado, White, Roaring Fork, _ Dolores, and San Miguel Rivers and Bear Creek. Some flooding occurred along the Colorado River downstream from the mouth of the Roaring Fork River. The peak discharge of the White River near Meeker (fig. 3. site 5) had a recurrence interval that exceeded 100 years. Peak discharge on Bear Creek at Morrison (fig. 3, site 1) occurred on July 22. The May and June 1984 floods also were the direct result of snowmelt combined with minor rainfall runoff. These floods had a larger areal extent than the floods of June and July 1983. Flooding was severe in the Yampa, White, Colorado, Roaring Fork, Gunnison, and Uncompahgre River basins. No official estimate of damage has been made for this flood, but damage was extensive in areas adja- cent to the rivers. Peak discharges for this flood were the maximum of record for the Colorado River near Cameo (fig. 3, site 4) on May 26 and the White River near Meeker (fig. 3, site 5) on May 25. The peak diSCharge at site 4 had a recurrence interval that exceeded 50 years and that at site 5 exceeded 100 years. DROUGHTS Major droughts were identified by analyzing cumulative de- partures from long-tenn average stream discharge at gaging stations operated since the early 1900's. Major droughts occurred during four 'C" '. periods-1930-42, 1949-57, 1958- . ,.JiP!!c'i: 70, and 1976-82. The areal extent . and severity of these droughts and the magnitude of annual departures from average discharge at the six selected gaging stations are shown in figure 4. The identification of drought periods is subjective because some gaging~ station records might show consis- tently less than normal annual de~ partures at the same time that other records show short -tenn greater than nonnal annual departures. This sub- jective differentiation of droughts also is involved in separating droughts when only about 1 year of intervening greater than normal an~ nual departures can be detected on most records, such as between the 1949-57 drought and the 1958-70 drought. In this instance, the droughts were separated because of the melting of an extremely large snowpack throughout most of the higher mountains in the State. In the (Pho- spring of 1957, the melting of this snowpack resulted in flood flows in Flooding along Elkhorn Avenue, Estes Park, Colo. during the dam failure on July 15, 1982. tograph courtesy of U.S. Bureau of Reclamation.l