Arkansas River Channel Capacity and Riparian Habitat Planning Study

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