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<br />2003 <br /> <br />increased from about 575 ft to 700 ft (23 percent) between 1870 and 1892. There were <br />considerable deviations from the average at individual cross sections, and widening did not <br />occur at every section. In this first reach the average width remained constant between 1892 <br />and 1926 although changes occurred at individual cross sections. Between 1926 and 1952 the <br />average width decreased significantly to an average of about 150 ft. No clear trend was <br />observed between 1952 and 1977. A similar widening and then narrowing trend was observed <br />in the second reach. Average bankfull width increased from 700 ft to 1160 ft (65 percent) <br />between 1870 and 1892. Significant increases in width occurred at each cross-section. Then <br />between 1892 and 1926 the average bankfull width decreased to 475 ft. Narrowing continued <br />at each cross section through 1952, when the average bankfull width was 180 ft., and through <br />19TI when the average bankfull width was 100 ft. In the third reach the 1870 average <br />bankfull width was reported to be 1230 ft. A continuous narrowing of the bankfull width was <br />reported for this reach. However, there is an error in the reported bankfull widths for 1892, <br />so that it is uncertain whether the actual average should indicate narrowing or widening. In <br />any event there was a definite narrowing trend continuing through 1926 (850 ft), to 1952 (450 <br />ft), and finally to 1977 ( 90 ft). <br /> <br />6. In addition to channel width changes, Nadler documented planform changes. <br />In reach 1 the river changed from braided to meandering. In reach 2 the river changed from <br />braided to a narrow straight stream. <br /> <br />7. Nadler attributed changes in the river channel to the following: <br /> <br />a. Irrigation improvements. This includes smoothing of the annual hydrograph due to <br />storage and release of irrigation water, and reduction in sediment load due to abstraction into <br />irrigation canals and due to John Martin Dam. Irrigation improvements caused the river to <br />become a perennial stream instead of an intermittent stream. Annual flow was also increased <br />due to diversions from other watersheds. <br /> <br />b. Introduction of salt cedar into the river basin in 1926, followed by a rapid increase <br />in population of this exotic species between 1936 and 1957. Vegetation along the banks resists <br />erosion and induces deposition. <br /> <br />c. The severe drought that occurred between 1924-1939 caused a decrease in mean <br />annual discharges, alJowing the establishment of vegetation along banks and in locations that <br />normalJy would have been below the mean high water level. At the same time irrigation <br />allowed for perennial flow and sufficient moisture to sustain dense vegetative growth. <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />5 <br /> <br />August 8, 1997 <br />