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<br />2.2 CLIMATE
<br />
<br />The climate in Prowers County is characterized as semi,aridlcontinental with low and
<br />variable precipitation, low humidity, and a wide seasonal range in temperature, Weather
<br />patterns generally are governed by dry air from the southwest; however, winter storms emanate
<br />from the northwest, and moist air from the Gulf of Mexico frequently influences weather
<br />during spring, summer, and falL
<br />
<br />During December through February, nighttime temperatures are usually below 32 of
<br />while daytime temperatures generally are above freezing, In June through August, the daily
<br />maximum temperature is 90 of or higher on about 70% of the days. The length of the freeze-
<br />free growing season in Prowers County is approximately 166 days (Pannell ef ai, 1966),
<br />
<br />Average annual precipitation at Lamar is about 15 inches, nearly 60% of which result
<br />from thunderstorms in May through August. The annual snowfall is about 24 inches (pannell
<br />ef ai, 1966), Because rainfall is low and summer tel):1peratures are high, evaporation rates are
<br />high during the growing season, Average annual pan (Class A) evaporation at John Martin
<br />Dam is 84 inches, of which approximately 66 inches occurs during April through October
<br />(USACE 1983), The months of November through March account for approximately 20% of
<br />the annual pan evaporation,
<br />
<br />2.3 PHYSIOGRAPHY, GEOLOGY, AND SOILS
<br />
<br />The Arkansas River in Prowers County lies within the High Plains section of the Great
<br />Plains physiographic province (Fenneman 1931) and is characterized by flat to gently rolling
<br />uplands with a few shallow valleys and many shallow, undrained depressions, The alluvial
<br />bottomland along the river ranges from one to three miles wide and is bounded on the north
<br />and south by terraces or sand hills, Slopes within the valley bottomland are less than 2%,
<br />
<br />Bedrock underlying the Arkansas River consists of Cretaceous sandstone, shales, and
<br />limestones, Most of the channel is within the Lower Cretaceous Dakota formation, composed
<br />of fine-grained sandstone and sandy shale, Saturated valley-fill alluvium consisting of gravel,
<br />sand, silt, and clay of Pleistocene to Holocene age and 50 to 150 feet thick occupies a trough
<br />eroded in the sedimentary bedrock,
<br />
<br />Soils within the Arkansas River floodplain belong to the Las-Glendive association
<br />which consist of alluvial materials that vary extremely in texture, depth, and drainage, These
<br />soils include sand and gravel, silt loams, and clay loams (Pannell ef at, 1966),
<br />
<br />2.4 WATER QUALITY
<br />
<br />The Arkansas River comprises Colorado's largest drainage basin, Originating in the
<br />Rocky Mountains in the central portion of the State, the river flows eastward for about 235
<br />miles before entering Kansas, The drainage area between John Martin Dam and Coolidge,
<br />
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