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1951. The recording stations show only the July 27, 1927, storm and that as nothing particularly severe. -, In summary, floods at Ouray have resulted from intense summer season rainfalls. No information has been uncovered that indicated snow melt presents a serious flooding problem in the Ouray area. c. Snowmelt Peak Flow Estimates for the San Juan Area Snow melt peaks rarely cause material flood damage in the San Juan Mountain area. Tn.e greatest peak flows have occurred outside of the snow melt season and have ranged from near the maximum snow melt peak to three or four times this amount. Local flood damage is much more likely to be caused by temporary debris jams than by lack of normal , channel capacity. However, the probability of high peak flows does increase with the snowpack. There is the probability that some combination of snow melt and rainfall will combine to produce flood problems from once in twenty to once in fifty years. The maximum flow recorded during the period 1936-67 for six major streams originating in the San Juan Range ranged from 188% of average for the Conejos near Mog.ot~ to 24.0% of av~ra~e for t!"1e.._ South Bork of the Rio Grande at South Fork" Colorado. ' Since the Conejos and main stream Rio Grande have substantial reservoir control the more practical limits are 200 to 240% of average. There were no excessive summer rainfalls covering a large area in the 1936-67 period although in 1941 and 1949 and to a lesser extent in 1957 summer rainfall (during the snowmelt season) was well above average. The maximum snow melt peak on these streams occurred in June, 1927, before snow pack records were available. The range was from about 300 to 500% of the average snow melt peak. Th'e maximum snow pack in this area occurred in 1952. All peak flows were less than should have been expected. The flows in this year should have been exceeded in 60 to 75% of the years. In 1952 there was some channel cleaning in the Dolores and some diking near the town of Dolores. The reservoirs on the Rio Grande were at low levels, due to three previous years of low flow. Complete filling was delayed. It is estimated that peak flow on the Rio Grande was probably reduced from 10,000 to 11,000 cfs to about 7,000. There was no material flood damage. Table XXIII shows the chances that specified percent of average peak flows will occur based in the 1936-67 records. Flows are going to exceed 200% of average about once in 15 years and 250% of 'average about once in 25 years. TABLE XXIII.--Probability of exceeding specified peak flows during snowmelt for the period 1936-67. Percent of Average Flow Stream 50 75 100 125 150 175 200 225 250 S. Fork Rio Grande at South Fork .98 .86 .50 .32 .20 . 13 .09 .07 .05 San Juan at Pagosa Springs .98 .80 .50 .38 .24 .14 .08 .06 .04 Animas at Durango .98 .80 .48 .28 . 17 .10 .06 .04 .03 Dolores at Dolores .98 .79 .49 .30 .19 .11 .07 .04 .03 Note: Rio Grande near Del Norte and Conejos at Mogote not included because of substantial regulation of flow. Extension of the probability curves indicate that 300% of average will be exceeded about once in 50 to 75 years. Streams like the Conejos which flows across a flat area in San Luis Valley where any peak flow near or above average could cause flooding of lowlands. This is one of the factors in operations of Platoro Reservoir- -to reduce peak flow to about 2000 cfs or 80% of average. The correlation coefficient betwe~n April 1 snow Viiater equivalent and peak flow are only in the range 0.3 to 0.7. The higher correlation,s exist on the high elevation gaging stations such as the San Juan at Pagosa Springs and the South Fork of the Rio Grande at South Fork. Correlation coefficients decline with the distance from the snow pack area. Typical scatter diagrams for snow water equivalent versus peak flow for the Animas at Durango and the Dolores River at Dolores are shown in Figures 60 and 61. A study was made of the probability of occurrence of certain flows from a specified snow pack on February 1 and April 1. The results are listed in Tables XXIV through XXIX. The period used was 1936 through 1967. Snow data had to be estimated for ~p to 5 years on February 1 for some streams. The }ollowing is the list of streams: 73