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The SOuth Arkansas River drainage basin above the downstream limit of the study encompasses approximately 135 square miles. Poncha Creek has a drainage area of approximately 60 square miles. The two drainage areas lie side by side on the east flank of the Conti- nental Divide. The South Arkansas River and Poncha Creek basins comprise the southern portion of the upper Arkansas River Valley in Chaffee and Lake Counties. The valley occupies a narrow and long trough that was formed mainly by faulting. The main topographic features of this valley are the two great parallel north-south mountain ranges that border the valley, the Sawatch Range on the west and the Mosquito Range on the east. The Mosquito Range merges with a low range of hills along the southern portion of the valley. The larger tribu- taries of the Arkansas River, including the South Arkansas River, originate in the Sawatch Range. A number of irrigation ditches, such as the North Pork Ditch, Cameron Ditch, and the Missouri Park Ditch, convey water across natural drainage boundaries into adjacent basins. The Larkspur Ditch conveys water from the headwaters of the TOmichi Creek drainage basin in the Colorado River basin across the Continental Divide at Marshall pass and into the poncha Creek basin. The elevation of the confluence of the South Arkansas River and Poncha Creek is 7,438 feet above mean sea level (MSL)'. The highest point in the South Arkansas River basin is 14,229 feet above MSL at Mt. Shavano. The highest point in the poncha Creek drainage basin is 13,955 feet above MSL, the top of Mt. Ouray. The South Arkansas River study reach has a defined channel with channel banks ranging from flat to steep slopes. The river reach meanders through the alluvial materials deposited by the streams and suggests a shift of the riverbed in the past in certain areas. The north bank, comprised of alluvial fans, slopes gently away from the river. The south bank, east of u.S. Highway 285, is comprised of long and narrow alluvial material bordered to the south by rough broken land consisting of sediments of the Dry union Formation. The south bank, west of u.S. Highway 285, is comprised of alluvial fans similar in nature to the north bank. The average slope of the study reach is approximately 0.01484. The poncha Creek reach is comprised of wet alluvial land in the lower one-half of the reach, with a steep, well-defined terrace in the upper one-half. The average slope of the study reach is approx- imately 0.0267. The study area climate is influenced by the westerly winds coming over the mountains. It is also influenced by less frequent but important moisture-bearing winds from the east during the summer. These climatic features are typical of a mountain valley on an 4