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<br />and the Kansas state line. Big Sandy Creek has the largest drainage area, 3,426 square miles. <br />However, this drainage area is characterized by significant valley storage, high infiltration, <br />numerous non-contributing areas, and several small Soil Conservation Service flood-retarding <br />structures. Flood peaks from the Big Sandy Creek watershed are not as great as would be <br />expected from its size. Major contributing drainage areas between John Martin Dam and the <br />Kansas state line are located south of the Arkansas River. These include Caddoa Creek, Mud <br />Creek, Clay Creek, Wolf Creek, and Two Butte Creek. <br /> <br />Sediment Accumulation in John Martin Reservoir <br /> <br />4. The reduction in sediment supply to the Arkansas River downstream from John <br />Martin Reservoir can be quantified using reservoir sediment survey data. Twelve surveys have <br />been conducted at variable intervals between 1942 and 1980. Total accumulated sediment <br />volume over the 38 year period is shown in Figure 2. The measured sediment accumulation is <br />compared to annual inflow of water for the same 38 years. The annual inflow is the sum of <br />the annual discharge from the USGS gages Arkansas River at Las Animas and Purgatoire River <br />near Las Animas. The figure indicates a decline in sediment accumulation after about 1968. <br />This corresponds to a general decrease in annual water runoff volume starting in 1966. This <br />figure can be extended by conducting a new reservoir survey, or by analytical methods. <br />Actual field measurements are preferred. Analytical methods should be verified using the <br />historical data. Extending the sediment accumulation data is recommended for the detailed <br />geomorphological assessment. Extending the sediment accumulation record and comparing the <br />results with annual runoff volumes will help identify long-term trends upstream from the dam. <br />These data can be used to establish a base condition downstream from the dam for comparison <br />purposes. <br /> <br />Historical Changes in the Streambed <br /> <br />5. Geomorphological changes on the Arkansas River between 1870 and 1977 were <br />reported by Nadler (1978). He identified changes in three distinctive reaches using historical <br />mapping through 1952 and measurements in 19n. The firSt reach was upstream from John <br />Martin Reservoir between La Junta and Las Animas. The second reach was downstream from <br />John Martin Dam, starting six miles upstream from Lamar and extending downstream for <br />about 22 miles. The third reach covered the first 20 miles upstream from the Kansas state <br />line. He compared bankfull widths at specific cross sections over time and calculated averages <br />for each reach. In the first reach, upstream from the reservoir; the average bankfull width <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />3 <br /> <br />August 8, 1997 <br />