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. County Reservoir Project investigation were adjusted to reflect the PMF at the Una dam site. .. The resulting Project PMF is estimated to have a peak flow of 130,000 cfs. . . The mean annual precipitation in the basin is approximately 16.9 inches and the 100- year 6-hour precipitation is estimated to vary from approximately 1.8 inches at Grand Junction to over two inches in the upper basin. The average 72-hour PMP over the drainage basin is 10.7 inches, based upon information provided in the September 1977 publication, Hydrometeorological Report No. 49, Probable Maximum Preci itation' Estimates 'Colorado RlVer an Great Basln Draina es HMR49. . . The peak flow for the PMF was calculated in two slightly different ways and then the results of the two methods were averaged. The average was then checked for reasonableness by three other estimation methods. The two primary estimation methods are based on: (1) adjustment of the port ion of the Tri -County PMF peak flow attributable to snowmelt by the direct ratio of the Una versus the Tri-County drainage areas above El 9000; and (2) adjustment of the snowmelt peak by the direct ratio of the average May 1 snowpack water equivalents in the two basins. This snowmelt peak was then added to the rainfall portion of the flood peak estimated by one of two methods as follows: The rainfall portion was calculated by multiplying the rainfall portion of the Tri-County PMF peak by the direct ratio of the PMP in each basin and then by either: (1) the direct ratio or (2) the square root of the ratio of the total drainage areas of the two basins. The peak flow is 121,000 cfs by the square root method and 138,000 cfs by the direct method. The average of approximately 130,000 cfs was adopted as the peak flow estimate for the PMF. . . The above adopted PMF est imate was then checked for general reasonableness by the following three methods: (1) est imate based on the rat io of the Tri -County PMF to the 500-year flood; (2) estimate based on the placement of the Tri-County PMF peak on the USBR peak discharge envelope curves used in the Dominguez Reservoir Flood Study and (3) comparing the estimate to that contained in the USBR Feasibil ity Report for the Bluestone Project on the Colorado River. The following peak discharge values were obtained based upon the above three methods, respectively: 125,000 cfs, 120,000 cfs, and 110,000 cfs. The peak discharge of 130,000 cfs for the PMF therefore appears conservative and has been selected for use in prel iminary design studies of the Una Project. The volume of the PMF was not estimated since it would be very large and no routing effect could be obtained 3686-a/5 1-10