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<br />-16. <br /> <br />.17- <br /> <br />The cyclic thermal input that occurs in the spring isat least <br />dS importdnt dS the depth of snowpack. This thermal input determines <br />the snowmelt rate and thus the streamflow discharges. The spring <br />snowmelt accelerates rapidly when the nightime temperatures remain <br />at or above freezing, but any cloud cover ~?mporarily interrupting <br />the thermal input results in a decrease in the melt rate, and thus, <br />the associated streamflow discharge rate. <br />Streamflow <br />The Gore Creek streamflows have an annual cycle. The high <br />discharges in the cycle, whiCh are caused by the annual snowmelt, <br />occur from late May to late June. Flows then decrease to less than <br />ten percent of the yearly peak by August and further decrease to the <br />yearly low in December or January. On the average, Gore Creek pro. <br />duces an allnual runoff of 24,000 acre-feet of water, of which 85% <br />is drawn off during this snowmelt portion of the annual cycle. <br />During the snowmelt season, the ground is partially sJturated <br />and still frozen. Consequently, the majority of the melted snow <br />enters the stream as runoff without significant infiltration losses. <br />The sllowmelt runoff is characterized by a diurnal fluctuation. <br />The annual peak discharge is caused by warm temperatures whiCh pro. <br />dute a higher diurnal fluctuation for the peak dischJrge dJy. This <br />peak is seldom much higher than those on preceediny or succeediny <br />days. as can be seen in Figures 111-3, 111-4 and 111-5, which show <br />the record discharges of June 1952. Flows usually remain very high <br />for one to three weeks, depending on mean daily temperature fluctu ations. <br /> <br />Examination of thunderstorm discharges to define summer precipi. <br />tation losses shows that a one-inch rainfall will yield approximately <br />0.1 inChes of runoff after late June (based on probable antecedent soil <br />conditions). In the spring months, much of the precipitation is retained <br />by the snowpack.1,2 These data compared favorably with those data Obtained <br />from a well-instrumented, but short-term study in the San Juan Mountains <br />of ColoradO, prepared by Loren ~. Crow, Consulting Meteorologist for <br />the Bureau of Reclamation.2 Comparisons of the Gore Creek Valley data <br />with the San Juan Mountain data indicate a slightly higher loss rate <br />In the Gore Creek Valley. The difference is caused primarily by a <br />lesser amount of rock exposure at Gore Creek. However, because of <br />the limited precipitation gaging in the Gore Creek Valley, the lower <br />loss rates of the San Juan study have been used. Thesevalueswlll <br />produce an excess precipitation of 0.17 inches from a one.inch one- <br />hour duration thunderstorm. <br />Since the thunderstorms in the high mountain areas are meteor- <br />ologically limited to areas of approximately ten square miles, the <br />only streamflow gaging stations that were expected to indicate high <br />stream flows from thunderstorms were stations located on smaller <br />basins. However, examining these data for the entire periods of <br />record failed to show any annual peak flood discharge caused by <br />rain. There are, however, spikes or small short-term increases on <br />the streamflow hydrographS after the more intense rainstorms. <br />The thunderstorm characteristics indicate that thunderstorm <br />runoff will be more severe than snowmelt runoff in very small water- <br />