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I I I merges with a low range of hills along the southern portion of the valley. The larger ~ributaries of the Arkansas River, including the South Arkansas River, originate in the Sawatch Range. , A number of irrigation ditches; such as the North Fork 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. I I I Except for a water-stage recorder at Marshall Pass which records trans- mountain diversions via the Larkspur Ditch, currently, there are no active streamflow gaging stations in any of the study area drainage basins. The historical streamflow records show that two stations have recorded flows 1n the study area. The first station, located on the South Arkansas River approximately a quarter of a mile upstream from its confluence with Poncha Creek, recorded flows during the 1911-1917 and 1971-1973 periods. The second station, located on Poncha Creek approximately a quarter of a mile upstream from its mouth, recorded flows during the 1912-1917 period. Stream- flow records from gaging stations downstream from the study area on the South Arkansas River and in the watersheds north of the study area are avail- able and were used in the hydrologic analysis portion of this study. I I I I I A basin map depicting the drainage basins and the location of the gaging stations is presented on Plate-I. I I Characteristics of Different Reaches I The study reaches for the South Arkansas River and Poncha Creek are shown on the Index Map, (Plate 2). The reach of the South Arkansas River begins at the Limit Street bridge and extends downstream approximately 1.42 miles, The Poncha Creek reach extends from its confluence point with the South Arkansas River upstream approximately 0.57 miles. I I I I -7- - I I I 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 river bed in the past in certain areas. The north bank compri- sed 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 boardered to the south by a 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. , I I I The Poncha Creek reach is comprised of wet alluvial land 1n the lower one- half of the reach; with a steep, well defined terrace in the upper one half. Old maps of Poncha Creek show its stream bed location to be different than its current location near its confluence with the South Arkansas River basin. The average slope of the study reach is approximately 0.0267. I I I The historical use of the floodplain has been for range and wildlife with limited improvements supporting ranching activities. Irrigated pastures on these plains have good plant cover and support meadow vegeta- tion. Obstructions of floodflows within the study reach include primarily natural obstructions such as vegetation growth, fallen trees, and to a lesser extent man-made structures such as bridges, buildings, and roadways. These obstructions have the tendency of increasing the flood elevations upstream of the obstructions and the velocities downstream of the obstruction, In certain instances, debris accumulated against a structure, does present a potential threat to the safety of the structure. ~ " I V , f, it ~ ~ I I I Climate The study area climate is influenced by the westerly winds coming over the mountains. It is also influenced by less frequent but important moist bearing winds from the east during the summer months. These climatic -8- I