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2.2 CLIMATE The climate in Prowers County is characterized as semi-arid/continental with low and variable precipitation, low humidity, and a wide seasonal range in temperature. Weather patterns generally are governed by dry air from the southwest; however, winter storms emanate from the northwest, and moist air from the Gulf of Mexico frequently influences weather during spring, summer, and falL During December through February, nighttime temperatures are usually below 32 of while daytime temperatures generally are above freezing. In June through August, the daily maximum temperature is 90 of or higher on about 70% of the days. The length of the freeze- free growing season in Prowers County is approximately 166 days (Pannell et al. 1966). Average annual precipitation at Lamar is about 15 inches, nearly 60% of which result from thunderstorms in May through August. The annual snowfall is about 24 inches (Pannell et al. 1966). Because rainfall is low and summer temperatures are high, evaporation rate:s are high during the growing season. Average annual pan (Class A) evaporation at John Martin Dam is 84 inches, of which approximately 66 inches occurs during April through October (USACE 1983). The months of November through March account for approximately 20% of the annual pan evaporation. 2.3 PHYSIOGRAPHY, GEOLOGY, AND SOILS The Arkansas River in Prowers County lies within the High Plains section ofthe Great Plains physiographic province (Fenneman 1931) and is characterized by flat to gently rolling uplands with a few shallow valleys and many shallow, undrained depressions. The alluvial bottomland along the river ranges from oI1e to three miles wide and is bounded on the north and south by terraces or sand hills. Slopes within the valley bottomland are less than 2%. Bedrock underlying the Arkansas River consists of Cretaceous sandstone, shales, and limestones. Most of the channel is within the Lower Cretaceous Dakota formation, composed of fine-grained sandstone and sandy shale. Saturated valley-fill alluvium consisting of gravel, sand, silt, and clay of Pleistocene to Holocene age and 50 to l50 feet thick occupies a trough eroded in the sedimentary bedrock. Soils within the Arkansas River floodplain belong to the Las-Glendive association which consist of alluvial materials that vary extremely in texture, depth, and drainage. These soils include sand and gravel, silt loams, and clay loams (Pannell et al. 1966). 2.4 WATER QUALITY The Arkansas River comprises Colorado's largest drainage basin. Originating in the Rocky Mountains in the central portion of the State, the river flows eastward for about 235 miles before entering Kansas. The drainage area between John Martin Dam and Coolidge, 10