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<br />-- <br /> <br />21 <br /> <br />178G <br /> <br />do not understand how a small change in one place can have extensive and significant <br />detrimental effects elsewhere. One of the goals of these planning studies is to educate the public <br />about how fragile a river sYstem is and, when disturbed by human intervention, that it may take <br />hundreds of years for the river system to regain equilibrium. <br /> <br />In reading the historic synopsis below, please consider the following statements. "When <br />the works of man run contrary to the natural, stable tendencies of the river, the river eventually <br />dominates. It appears then to be of monumental importance to understand the interrelated <br />process variables which shape the dimension, pattern and profile of the modern river" (Rosgen <br />1996: 111). "Natural rivers, which are self-constrUcted and self-maintained, constantly seek their <br />own stability" (Leopold et al. 1964 in Rosgen 1996:112). A "stable channel balance" is a <br />generalized relationship of the proportionality between sediment discharge, stream discharge, <br />particle size and slope (Lane 1955 in Rosgen 1996:212). "Whenever proper attention to the <br />"rules of the river" is not respected, adverse channel adjustments often result in damage to <br />personal property and loss of life" (Rosgen 1996:1/3). <br /> <br />A Review of Phvsical Characteristics and Historic Events <br /> <br />In the study area, the Arkansas River Valley is characterized as a broad valley with slightly <br />undulating uplands on either side. The valley gently slopes to the east with the floodplain <br />bordered by rather extensive Quat~rnary age gravel terraces (Fenneman 1931:35-37; Sharps <br />1969; USACE 1983a:4/1-412). <br /> <br />"Sandy and gravelly deposits of late Pleistocene age occur on the lower terraces <br />along the Arkansas River. The surfaces of these terraces usually lie from 30 to 80 <br />feet above the present stream channel. These terrace deposits are generally mantled <br />with younger eolian and alluvial deposits, which are the parent material for a large <br />part of the irrigated land. Along the Arkansas River, the maximum thickness of the <br />Pleistocene valley fill is about 50 feet. It is a reservoir for ground waler and is <br />tapped by numerous irrigation wells" (Elkin 1972:72). <br /> <br />John Martin Dam is situated in the Arkansas River Valley on the Las Animas Arch, a <br />syncline where Dakota Sandstone is exposed at the swface (Sharps 1969:C69; Holliday <br />1982:383; Van Couvering 1982:402). The Dakota Sandstone is structurally sound and at this <br />point in the valley confines the river (Holliday 1982:393), thereby providing the best location for <br />the construction of a dam. <br /> <br />Morphologically, Sharps (1969) provides a good description for the formation, during <br />Quaternary time, of the valley's gravel terraces that occur adjacent to the river and of the river's <br />lateral migrations. <br /> <br />A...the Arkansas Ri ver between Fowler and La Junta, Colo., has migrated northward <br />as much as 7 miles, and between La Junta and Kansas it has migrated southward as <br />much as 9 miles" (Sharps 1969:C66-C67). AAtlhe end of lilinoian or Sangamon <br />time [approx. 70,000 years before present] the river had cut down to the top of the <br />competent Dakota Sandstone at the crest of the Las Animas arch in the vicinity of <br /> <br />11 <br />