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<br />. <br /> <br />8. Thirteen sediment ranges have been established downstream from John Martin Dam <br />by the Corps of Engineers. These ranges extend about 23 miles to Lamar, The ranges have <br />been surveyed at varying intervals between 1944 and 1987. In general, the surveys indicate <br />degradation and narrowing of the river channel. Degradation of the river thalweg averages <br />about 3 feet for the first 16 miles downstream from the dam. Aggradation occurred at ranges <br />located 18 and 20 miles downstream from the dam. This may be related to a cutoff that <br />occurred upstream from these two ranges. Degradation was recorded at the final range which <br />is located 23 miles downstream from the dam. The surveys also indicate a narrowing of the <br />channel caused primarily by deposition irr the old channel. The surveys also indicate that this <br />channel adjustment occurred over a period of years, moving steadily downstream. The surreys <br />indicated the following: <br /> <br />a. Through 1951 the first eight miles downstream from the dam adjusted by deposition <br />in a portion of the main channel and degradation in another part of the channel <br />resulting in a narrower and deeper channel. The remaining fifteen miles adjusted <br />primarily by aggradation on the sides and/or in the middle of the channel. The most <br />downstream ranges adjusted only slightly. <br />~ <br />b. Deposition'.on:j1e sides and/or middle of the channel accompanied by degradation <br />continued through 1962 in the first eight miles downstream from the dam, and the same <br />trend was initiated in the next seven miles. The remaining eight miles adjusted <br />primarily by aggradation. <br /> <br />c. During the next four years, through 1966, trends for deepening and narrowing <br />continued in the first fifteen miles. Downstream for the next eight miles the channel <br />adjusted primarily by aggradation on newly formed bars or along the channel sides <br />resulting in a narrower channel. <br /> <br />d. The 1972 survey indicated a trend reversal at most of the ranges. Aggradation <br />occurred throughout the 23-mile reach. At ranges upstream and downstream from <br />Caddoa Creek (located 2.5 miles downstream from John Martin Dam) the channel <br />aggraded back to pre-dam elevations. <br /> <br />e. The last survey was taken in 1987 and indicated that the 1972 changes were <br />temporary. The channel was generally narrower and deeper than in 1966 for the 17 <br />miles downstream from the dam. The range located about 18 miles downstream from <br />the dam remained an anomaly. It aggraded significantly between 1951 and 1962 and <br />thereafter remained essentially stable, showing no trend for narrowing. The two <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />6 <br /> <br />August 8, 1997 <br />