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"= -17- Examination of thunderstorm discharges to define summer precipi- tation losses shows that a one-inch rainfall will yield approximately 0.1 inches of runoff after late June (based on probable antecedent soil conditions). In the spring months, much of the precipitation is retained by the snowpack.1,2 These data compared favorably with those data obtained from a well-instrumented, but short-term study in the San Juan Mountains , ., of Colorado, prepared by loren W. Crow, Consulting Meteorologist for the Bureau of Reclamation.2 Comparisons of the Gore Creek Valley data with the San Juan Mountain data indicate a slightly higher loss rate in the Gore Creek Valley. The difference is caused primarily by a lesser amount of rock exposure at Gore Creek. ~owever, because of the limited precipitation gaging in the Gore Creek Valley, the lower loss rates of the San Juan study have been used. These values will produce an excess precipitation of 0.17 inches from a one-inch one. hour duration thunderstorm. Since the thunderstorms in the high mountain areas are meteor- ologica11y limited to areas of approximately ten square miles, the only streamflow gaging stations that were expected to indicate high stream flows from thunderstorms were stations located on smaller basins. However, examining these data for the entire periods of record failed to show any annual peak flood discharge caused by rain. There are, however, spikes or small short.term increases on the streamflow hydrographs after the more intense rainstorms. . The thunderstorm characteristics indic~te th~t thunderstorm runoff will be more severe than snowmelt runoff in very small water- ,- , -18- . sheds. The Gore Creek catchment area below Black Gore Creek ranges from 34 to 100 square miles. In these larger areas, the snowmelt discharges are significantly higher than are the rainfall runoff discharges. In contrast, the general storm will cover the entire basin and should produce its greatest effect on Gore Creek near the Eagle River. However, the general rainstorms precipitate at such a low rate that the infiltration absorbs most of the water. Breaking the 100 year I , general storm into time increments in accordance with the recommend ations of the "Design of Small Dams" results on 0.44 fnches of precipitation in the most intense hour.19 The peak intensity during this hour is unlikely to be greater than the O.B inch per hour infiltration rate.1 Additionally, portions of this precipitation will also fall as snOw or hail pellets which will further delay runoff. Gore Creek Flood Peak The U.S.G.S. streamflow gages provide a high-quality record of the runoff rates and water yields.20 Within the Gore Valley, there are ten st.reamflowgages. Two of these have just been installed. Theothers have periods of record from eight to twenty years, see Figure 111-6. In addition to these eight stations, streamflow records at six other locations with Similar orientation and orographic character were analyzed. The six additional stations have periods of record rang1ng from fourteen to forty-five years. The catchment areas for these streamfl~d g~ges range in size from 4.37 to 650 square miles. The streamflow records from the eight Gore basin stationS plus the six similar basin stations were statistically analyzed using a Log.Normal relationship as well as a Log-Pearson III relationship for .