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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />,- <br />.: <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br /> <br /> <br />19 <br /> <br />1784 <br /> <br />temperature is 90 OJ' or higher on about 70 percent of the days. The length of the freeze-free <br />growing season near La Junta in the Arkansas River Valley is approximately 160 days (Berry <br />1972:79). <br /> <br />Average annual precipitation in the study area is about 12 inches, nearly 60 percent of <br />which result from thunderstorms in May through August (Berry 1971:2, 1972:77-79). The <br />annual snowfall is about 20 inches (US ACE 1965:17). Because rainfall is low and summer <br />temperatures are high, evaporation rBtes are high during the growing season. Average annual <br />pan (Class A) evaporation at John Martin Dam is 84 inches, of which approximately 66 inches <br />occurs during April through October (USACE 1983a:4/4-4/5). The five months, from November <br />through March, account for approximately 20 percent of the annual pan evaporation. <br /> <br />2.3 PHYSIOGRAPHY, GEOWGY, AND SOILS <br /> <br />The study area upstream of John Martin Dam lies along the border of the Colorado <br />Piedmont and Raton subsections of the High Plains section of the Great Plains Physiographic <br />Province (Fenneman 1931:6, Figure 2, 30-47; Abbott 1985:5, Figure 2; USACE 1983a:412). The <br />region is characterized by flat to gently rolling uplands with a few shallow valleys and many <br />shallow, un-drained depressions. The alluvial bottomland along the ri ver ranges from one to <br />three miles wide and is bounded on the north and south by gravel terraces and sand hills. <br /> <br />Bedrock underlying the Arkansas River consists of Cretaceous sandstone, shale, and <br />limestone that have "a very slight regional dip to the north-northeast of about 0.5 degrees to 3 <br />degrees" (Van Couvering 1982:397). In the John Martin Dam area, most of the channel is within <br />the Lower Cretaceous Dakota fonnation that is composed of fine-grained sandstone and sandy <br />shale. Saturated valley alluvium consisting of gravel, sand, silt, and clay of Pleistocene to <br />Holocene age averages about 50-feet thick and occupies a trough eroded in the sedimentary <br />bedrock (Fenneman 1931 :22; Elkin 1972:72; Watts and Linder-Lunsford 1992:5). The gradient <br />in the river valley below Pueblo varies between 5 and 9 feet per mile (USACE 1973:34; <br />1977a:3). <br /> <br />Soils within the Arkansas River floodplain belong to the Rocky Ford-NumB-Komman and <br />the Las-Apishipa-Bankard associations that consist of alluvial materials varying extremely in <br />texture, depth, and drainage (Larsen el al. 1972; Preator et al. 1971; USACE 1983a:4/2). These <br />soils include sand and gravel, loams, clay ]oams, and silty-clay loams. <br /> <br />2.4 WATER RESOURCES <br /> <br />The Arkansas River comprises Colorado's largest drainage basin. Originating in the Rocky <br />Mountains in the central portion of the State, the river generally flows southward for about 60 <br />miles through mountain valleys and then eastward across the plains for about 240 miles before <br />entering Kansas. The drainage area above John Martin Dam is 18,915 square miles of which <br />18,130 square miles are contributing drainage area (USACE 1983a:4/1; 1984:2). Snowmelt in <br />the upper reaches of the basin generally begins in April, with the majority of runoff occurring <br />from May through July. <br /> <br />9 <br />