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March (these would produce increases). At Wolf Creek there was an average of 17% greater snow accumulation during the seeded 1965, 1967, and 1969 seasons than during the previous eight year period, including the randomized non- seeded years of 1966 and 1968 in relation to the snow at upwind control stations. Again, it must be remembered that the seeding was carried out under meteorological stratifications that could be expected te have produced both increases and decreases in precipitation. These studies of the use of snow course reading for evaluation of weather modification are just gettiIg underway and are being continued. It is not expected that snow course data will prove very useful for weather modification evaluation for reasons presented above. 5. Investigation of Flood Potential Resulting from Snowmelt a. General discussion On Colorado River Tributaries most of the annual peak streamflow comes during the highest snow melt runoff, usually in Mayor June. In rare years annual peaks may occur in late April on a few tributaries or in early July in a heavy snow year when weather factors combine to delay snow melt. Snow melt peaks rarely are responsible for material flood damage. These annual peaks seldom vary more than about 5 ft in actual stage and overflows that do occur are over meadow or unoccupied land near the rivers. Snowmelt peak flow is moderately well related to total snow melt season flow. In the few instances where relationships between snow melt peak flow and total snow melt season flow have been compared, the correlation coefficient ranged between 0,50 8,nd 0.80. Temperature sequences have a substantial effect on rate of snow melt runoff, as well as total snow pack. The area of snow covered ground is the other major factor in affecting snow melt rates. The area of snow covered ground at any specific date during the snow season tends to be well related to the relative depth of the snow pack of the preceding season. The highest recorded peak flows for San Juan and Rio Grande tributaries in southwestern Colorado and adjacent streams occurred on October 5, 1911, a date well after the end of the snowmelt period. These peaks were two to three times the volume of typical snow melt peaks and obviously was the result of heavy rains. There are several other records on streams rising in the San Juans where annual peak flows occurred in late July or during August, also well past the snow melt peak. These other records of peak flow were in the general range of maximum snow melt peak flows at the gaging stations. It is suspected that flows were much higher on smaller tributaries draining the watersheds where the isolated storms occurred. Peaks outside of the snowmelt season occur about once in te.n years in the San Juan Mountains area and less frequently on other Upper Colorado River watersheds. These periods of intense flow produce extreme peaks but do not produce substantial portions of the annual flow since they are of short duration. I b. Flooding in Ouray, Color4do, Area There is a history 6f flooding in the Ouray, Colorado, area but there ~s limited record of high flows at regular gaging st~tions. I I There was a gaging fltation in operation at Ouray, including the Uncompah'gre and Canyon Creek, for the period 1914-29. ~he station at Ridgeway was established in 1957: This station is about 10 miles downstream from Ouray. They are I probably comparable as to total flow but have very little relation as to peak flow. : The highest flow recorded at the Ouray Station was on June 11, 19:31 at 2400 cfs at a gage height of 6. 1 ft. The highest gage height recorded was 13.3 ft on July 27, 1927, because of a log jam. There have no doubt been other unrecorded log or ice jams in the area. A few other streams in the area have records of above b~nkful stages in winter months. The 1927 water year was one of high runoff, probably exceeded four or five times in the past 50 years, aDn then by less than 150/.:,. The maximum flow which has occurred since the turn of the century was probably on October 5, 1911, the above mentioned period of high rainfall in the area. I The records of major floods in Ouray, are based ,on limited flQPd surveys and newspaper accounts. The most recent seve~e flood occurred on July 11, 1965. The flood was cau;sed by rainfall on small drainages of one to three square miles near town on steep slopes (Portland and Cascade Creeks). The rainfall occurred between 6:~0 p. m. and 11:00 p. m. The peak occurred about midnight. The flow through Ouray was estimated to be in excess of 8000 cfs. There is no indication bf how the estimate was made or the authority for making the estimate. I The flood damage was caused by ~ud and reck slides blocking the creeks at the highway bridge and the clogging of a flume (apparently a by-pass) in the downtown area. Damage was esUmated at $200,000. The precipitation I1ecord at Ouray shows only. 04 inch on July 11; O. ~6 on July 12; and 0.51 on July 13. The newspaper k.ccount credits the flood to a cloudburst on the s 19pes above town. The account also mentions rain hampering cleanup on July 12 and 13. The gaging station 00 the Uncompahgre at Ridgeway, 10 miles downstream, recorded a daily peak of 660 cfs qn July 11, rising to 1020 cfs on July 12 and recediqg to 880 cfs and 631 cfs on July 13 and 14, respectively. If the peak flow did reach 8000 cfs which must be questioned, it would have been of very short duration. The only remote contribution from snow melt to this flood was that the peak flow was relatively late in 1965. It occurred on June 20 at 1200 cfs at Ridgeway. The year 1965 was a moder- ately heavy snow year, exceeded about 3 years in 25. Watershed soils were still saturated, a near normal situation. With the apparent rain intensity soil moisture conditions were a minor factor. A similar storm occurred on July 25, 1929 (possibly a more severe storm), and storms of lesser magnItude were reported by the newspaper on August 22, 1909, July 27, 1927, and August 2, 72