Arkansas River Channel Capacity and Riparian Habitat Planning Study

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do not understand how a small change in one place can have extensive and significant detrimental effects elsewhere. One of the goals of these planning studies is to educate the public about how fragile a river system is and, when disturbed by human intervention, that it may take hundreds of years for the river system to regain equilibrium. In reading the historic synopsis below, please consider the following statements. "When the works of man run contrary to the natural, stable tendencies of the river, the river eventually 1 dominates. It appears then to be of monumental importance to understand the interrelated process variables which shape the dimension, pattern and profile of the modem river" (Rosgen 1996:1/1). "Natural rivers, which are self- constructed and self maintained, constantly seek their ' own stability" (Leopold et al. 1964 in Rosgen 1996:1/2). A "stable channel balance" is a generalized relationship of the proportionality between sediment discharge, stream discharge, particle size and slope (Lane 1955 in Rosgen 1996:2/2). "Whenever proper attention to the ' "rules of the river" is not respected, adverse channel adjustments often result in damage to personal property and loss of life" (Rosgen 1996:1/3). ' A Review of Physical Characteristics and Historic Events In the study area, the Arkansas River Valley is characterized as a broad valley with slightly ' undulating uplands on either side. The valley gently slopes to the east with the floodplain bordered by rather extensive Quaternary age gravel terraces (Fenneman 1931 :35 -37; Sharps 1969; USACE 1983x:4/14/2). ' "Sandy and gravelly deposits of late Pleistocene age occur on the lower terraces along the Arkansas River. The surfaces of these terraces usually lie from 30 to 80 feet above the present stream channel. These terrace deposits are generally mantled with younger eolian and alluvial deposits, which are the parent material for a large part of the irrigated land. Along the Arkansas River, the maximum thickness of the Pleistocene valley fill is about 50 feet. It is a reservoir for ground water and is tapped by numerous irrigation wells" (Elkin 1972:72). John Martin Dam is situated in the Arkansas River Valley on the Las Animas Arch, a syncline where Dakota Sandstone is exposed at the surface (Sharps 1969:C69; Holliday 1982:383; Van Couvering 1982:402). The Dakota Sandstone is structurally sound and at this ' point in the valley confines the river (Holliday 1982:393), thereby providing the best location for the construction of a darn. ' Morphologically, Sharps (1969) provides a good description for the formation, during Quaternary time, of the valley's gravel terraces that occur adjacent to the river and of the river's lateral migrations. ' A ... the Arkansas River between Fowler and La Junta, Colo., has migrated northward as much as 7 miles, and between La Junta and Kansas it has migrated southward as ' much as 9 miles" (Sharps 1969:C66 -C67). AAt the end of Illinoian or Sangamon time [approx. 70,000 years before present] the river had cut down to the top of the competent Dakota Sandstone at the crest of the Las Animas arch in the vicinity of 21 1 11