Arkansas River Channel Capacity and Riparian Habitat Planning Study

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!7 John Martin Reservoir. Since that time the elevation of the top of this sandstone has ' controlled a temporary base level that has slowed downcutting upstream from the arch. This slowing of downcutting has caused the river to meander widely along the ' section between Adobe Creek and the arch" (Sharps 1969:C68 -C69). Sharps (1969:C70) attributed the lateral migration of the river to the disproportionate deposition of large volumes of finer sediments from the river's tributaries. Recent studies indicate that these geologic sedimentation processes, that involve. the deposition of large amounts of sediment to the main channel, are still active today (USACE 1965:23, 1970 :B65 -B66, 1973:50 -53, ' 1985:15; Nadler 1978:89, 113, 126; Nadler and Schumm 1981:104, 109, 113). The Las Animas arch may also be responsible for the location of what has been described ' as the famous "Big Timbers" (Grinnell 1923:82-85; Wolf 1932 :207; West 1995:26 -27, €rontpiece map). Big Timbers, located downstream bf John Martin Dam and the arch, extended to the vicinity of Lamar and was the location of a large concentration of cottonwood trees, many of which grew to huge sizes (Grinnell 1923:82 -85; Wolf 1932 :207). The Big Timbers was a noted resting stop on the Santa Fe Trail and was also extensively used by Native American ' during the winter months. The valley area below the arch widened and flood flows constricted by the arch most likely rushed through the Las Animas Arch flowing out of the channel to overbank areas and spread over the widening valley.; This annual event would have replenished ' the soil nutrients in this area and afforded the continued growth of a healthy and extensive cottonwood forest. Another interesting feature of the valley, located on the south side of the river, beginning ' east of La Junta and reaching into central Kansas, is "the most striking sand -dune area of Colorado" (Fenneman 1931:34). Eolian processes cbnsisting'of hard northwest winds blew the lighter sands out of the river's floodplain to forth the dunes (Elkin 1972:72; Fenneman 1931 :34; Holliday 1982:394 -396). These "lighter sands" are also indicative of the sediments that are being transported by stream flows and deposited in the Arkansas River channel today. I The Arkansas River is an alluvial, sand -bed river with a "stream course [that] is characterized by low banks and a broad sandy bed, which shifts frequently (USACE 1983a:4/1- 4/2, 4/8 -4/10; 1985:3; Watts and Lindner - Lunsford 1992:4). Historically the river, in the study ' area, was a straight braided stream; however the river has undergone significant changes over the last 150 years and most significantly since 1926 (Nadler 1978:117; Nadler and Schumm 1981:111). Nadler (1978) conducted an extensive study on the Arkansas River that included ' analysis of channel morphology such as width, dept}, width -depth ratio, meander wavelengths and amplitude, and gradients along with hydrology, vegetation, and sediments. Nadler (1978) and Nadler and Schumm (1981:95) indicate that the Arkansas and the South Platte Rivers ' "...have undergone dramatic historic changes that are so extensive that they can be termed a metamorphosis." , Nadler (1978:58 -60) and Nadler and Schumrii (1981:95 -97) use the 150 -year time period which began with the 1849 California and 1859 Colorado gold rushes to pin -point a historical ' reference point from which to reflect upon the significance of the introduction of intensive agriculture and the subsequent development of irrigation systems in the Arkansas and South