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. . . Interdisciplinary Paleoflood Investigation of the Elkhead River Basin and Vicinity near Craig, Colorado Robert D. Jarrett, Hydrologist U.S. Geological Survey, WRD National Research Program P.O. Box 25046, MS 412 Denver, CO 80225 RECEIVED NOV 04 1996 Colorado '"Vater ' Conservation 80ard ABSTRACT Since about 1950, the design criteria for dams has included an estimate of the probable maximum flood (PM F), which is derived from the probable maximum precipitation (PMP). Accurately estimating maximum bounds of extreme precipitation and flooding requires a delicate balance between cost and safety. The National Research Council (1994) recognized that the greatest deficiency of the PMP/PMF is for extreme stonns in mountain basins less than about 400 mi2 and the need for regional analysis of extreme precipitation and f100di~g. A paleoflood study was conducted for the Elkhead River Basin, which is a tributary to the Yampa River near Craig, Colorado, Elkhead Reservoir and Dam are located at an elevation of about 6,400 fl; the drainage area upstream from the dam is 205 mi2. The PMF for Elkhead Reservoir is 36,052 ft3/s (Ayres Incorporated, written commun., 1996). The objective of this study was to detennine the magnitude of maximum rainfall and flooding that has occurred in and near Elkhead Creek Basin. An interdisciplinary paleoflood methodology was used in northwestem Colorado, which is a homogeneous hydrometeorologic region, to provide additional supporting infonnation on maximum- flood potential in Elkhead Creek Basin. The study area included the Yampa River Basin above Maybell and the White River Basin above Meeker, The methodology included onsite paleoflood investigations to detennine maximum paleoflood magnitude and age and analysis of extreme-precipitation and flood data. The primary paleoflood evidence used as indicators of past flood levels was bouldery flood-bar deposits, which can last 10,000 years or longer. Paleoflood estimates, which have an estimated error of about +/- 25 percent, were made for 93 sites, including a number at long-tenn gaged sites. No evidence of substantial out-of-bank flooding in at least 10,000 years was found in any investigated stream in northwestem Colorado, The maximum paleoflood in the Elkhead River varies from 1,800 to 3,300 fl3/s at the upstream from Elkhead Reservoir to about 4,750 ft3/s downstream from the reservoir. For comparison, the maximum paleoflood for the Yampa River is about 10,000 ft3/S at Sleamboat Springs and about 31,000 ft3/s near Maybell gage. Flood evidence was found related to an intense ralnstonn in the spring of 1995 on Fortification Creek at Craig, which appears to have produced the second largest flood; the largest in historic times occurred in 1984, The 1995 peak discharge was about 1 ,300 fl3/s and the 1984 peak was about 2,800 fl3/s. According to residents in Craig and onsite investigations, the 1995 rainstonn primarily occurred over about 30 mi2 of lower Fortification Creek Basin. 1