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TABLE XXI. --Continuous stormy periods at Wolf Creek Pass that resulted in total precipitation exceeding 3. 5 inches water equivalent at Wolf CrE:ek West and/or Wolf Creek Summit. W. C. Summit W. C. West Precip. Precip. Winter Season Period Treatment (Inches) (Inches) 1964-65 Dec. 24-28 Seeded 4.02 M Jan. 5-8 Seeded 3.57 4.86 April 2-15 Seeded 4.89 M 1965-66 Nov. 23-27 Not Seeded M 3.89 Dec. 22-25 Not Seeded 3.26 M Dec. 3)-Jal1. 2 Not Seeded 3.21 M 1966-67 Dec. 4-E: Seeded 6.21 5.36 1967-68 Dec. 13-21 Not Seeded 7.52 7.83 Jan.27-FEb.1 Not Seeded M 3.89 on the high elevation tributaries with the heavie::;t annual snow packs. Typically, flows are within 20"/0 of average about one-half the time. Extreme flows range from 30 to 175"/0 of average. From a hydrologic standpoint, the best areas for attempts to increase runoff through weather modification are the areas of relatively heavy snow pack. In Colorado, these areas include the San Juan Mountain range, the headwaters of the Roaring Fork and Crystal Rivers, the headwaters area of the Upper Colorado including the Blue River and the Park Range tributaries to the Elk and Yampa Rivers. The Grand Mesa area of west central Colorado is a limited size but is also an area of relatively heavy snow pack. Most of the surface water developed in the Upper Colorado River Basin in Colorado (indud- ing the San Juan and Green River Tributaries) flows out of the state. In the past 20 years it is estimated that an average of about 10 million acre-feet wa.s produced in the Colorado River Basin in Colorado. Of this amount about 4"/0 was delivered to other basins through trans -mountain diversions, and about 14"/0 was consumptively used within the Basin. The principal diversions to other basins include the Colorado-Big Thompson through Adams tunnel, the Twin Lakes Tunnel diversion from the Roaring Fork to the Arkansas, the Moffatt I Tunnel diversion by the city of Denver from the Fraser to Boulder Creek, and the recently completed Blue River diversion by the city of Denver. ThE: latter diversion had little effect on long term averages. For the Green River in Wyoming, there are no diversions to other basins. It is roughly estimated that less than 10"/0 of the water generated in Wyoming is comsumptively used in that state. In Utah, much of the wa ter produced within the basin is taken out by trans-mountain diversions or is con- sumptively used. A rough estimate is that one-half , of the flow generated within the Utah basins reaches the Green or Colorado Rivers. Flows of less' than 100,000 acre feet a year are generated on the water- . shed of the Colorado River in New Mexico. 2. Snowpack-Runoff Characteristics Snow water equivalent measured :::,t mountain snow courses and winter precipitation measurements at high elevations have been the principal factors used in forecasting snow melt season flow in western streams for up to 60 years. The major effort in making streamflow forecasts started some 35 years ago. Forecasts are now made for all snow melt streams of the west. Extensive application of these forecasts has been made to various water management operations. The characteristics of snowpack and streamflow relationship are important to various aspects of a program of weather modification for increasing water supplies. The quality of these relationships largely determines their usefulness for evaluating seeding effects, making conversions of measured precipitatim changes to quantities of streamflow added, determining flood threats that might be accentuated from weather modification, etc. A study has been made relating April 1 snow water equivalent on selected snow courses to April-August stream flow for 70 representative stations in the Upper Colorado River Basin. Streams in Colorado, Wyoming and Utah have been included. ' The selection of individual snow courses tobe used was based on high correla- tion with runoff, a logical location in or near the watershed, and an adequate length of record (also runoff record). The longest record used was 20 years. The use of this length of record minimizes problems of the continuing changes in data gathering procedures and in water use practices. Snow course data is subject to ,gradual change because of changes in overhead interception and general exposure of terrain. There are at times drastic changes in the data sites caused by timber cutting, fire, wide- spread timber disease and construction activities. Snow courses affected by material change were not used in these comparisons except in perhaps two instances and these are not considered as extreme. Some snow, courses have much better correlation with runoff than others. The reason is not always apparent but it generally is related to high elevations and a protected sampling area. Runoff records are affected by changing patterns of water use as well as improvements in water measuring techniques. Some cf the stations in these comparisons are affected by trans-mountain diversions and water use above the station where measurements of diversions 63