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United States Department of the Interior CS GEOLOGICAL SURVEY Sm.. 2';04(, ~1.S 'T-' 1..: Dmv~r h~derJ.J Center Denver, Colorado 80225 February 1, 1996 RECEIVED FES 05 1996 ]" Ri:I'~ Y RFFER Tn Mr. Ray D. Tenney, P.E. Colorado River Water Conservation District P.O. Box 1120 Glenwood Springs, Colorado 81602 Colorado Water Conservation Board Dear Mr Tenney: This letter is in response to our telephone conversation about mid-October 1995 conceming the use of the June 3, 1952 rainstorm of 5.25 inches measured in a U.S. Weather Bureau precipitation gage located about 3 miles southeast of Cimarron, Colorado in the site-specific probable maximum precipitation (PMP) analyses by North American Weather Consultants for Elkhead Reservoir near Craig, Colorado. A rainstorm of 5.25 inches in about 1.5 hours at an elevation of 6,900 feet is one of the largest reported rainfalls in westem Colorado. The use, reliability, and transposition of the 1952 Cimarron storm in extreme precipitation studies in Colorado has been debated for many years. For severO'1 years, I had wanted to conduct a paleoflood investigation to help assess the accuracy and validity of this rainstorm, which is but one of a number of questionable extreme rainstorms in Colorado that need to be evaluated. Because of the general importance of the 1952 storm to flood hydrology and specifically its use in the Elkhead Reservoir PMP study, a paleoflood study of the Cimarron area wa:; incorporated into my paleoflood investigation of the Elkhead River basin. We also conducted a paleoflood onsite visit for another notable large rainstonn recorded on Wolf Creek Pass in August ~ 995 (4.05 inches in 24 hours on the 20th). That evaluation indicated that no substantial rainfall runoff was recorded at any stream draining Wolf Creek Pass during that period (infonnal evaluation attached). Our review of rainfall, streamflow, and paleoflood investigations is enclosed. It has receivdd technical review from meteorologists and hydrologists as noted below. Although it is difficult 43 years later to categorically state that the June 3, 1952 rainfall of 5.25 inches at Cimarron 3SE is ar, erroneous measurement, we believe that an analyses of hydroclimatic data and paleoflood investigHiions clearly demonstrate that it is at least highly questionable if not erroneous. These analyses indicate that perhaps only about 0.5 inches occurred on June 3,1952. This conclusion is supporte'J by an independent evaluation of meteorologic data for June 3, 1952 by Henz Meteorological Services in Denver, Colorado. No significant rainfall runoff was recorded on June 3, 1952 at the U.S. Geological Survey streamflow-gaging station, which was fortuitously located about three miles downstream from the Cimarron 3SE precipitation gage. On June 3rd, the base-flow snowmelt runoff was about 795 cubic feet per second, and the total snowmelt and rainfall runoff peak flow was about 865 cubic feet per second. Thus, the net increase from rainfall runoff was about 70 cubic feet per f,econd or 9 percent of the antecedent snowmelt discharge on June 3rd. A U.S. Geological Survey hydrographer, who maintained the gage, was at the gage on June 4th and 5th, 1952 and noted the gage was operating correctly. The hydrographer made no comment of any extraordinary rainfall or runoff as normally is done. Nor did the rain gage observer make any mention that the June 3rd rainstorm was particulariy noteworthy. Fifteen other rainfall-runoff events were recorded at the Cimarron River gage in 1952, which corresponded to less than an inch of daily rainfall at Cimarron 3SE. The largest net rainfall runoff in 1952, which occurred on August 13th, had a peak discharge of about 168 cubic feet per second corresponding to D.5-inches of rain that was recorded at the Cimarron 3SE precipitation gage. For comparison, in 1951 there were 11 rainfall-runoff events. The largest was 588 cubic feet per second, which occurred on August 2nd and 3rd. The maximum paleoflood d;scharge in the Cimarron River at Cimarron ranges from about 3,000 to 4,000 cubic feet per second and was estimated from onsite paleoflood investigations. That paleoflood is believed to be associated with either the 1957 or 1984 snowmelt runoff; both years are recognized as having been near to or record snowmelt runoff in many