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24 I I I I I I I I I I I I I I I I I I I b. Introduction of salt cedar into the ,river basin in 1926, followed by a rapid increase in population of this exotic species between 1936 and 1957. Vegetation along the banks resist~ erosion and induces deposition. c. The severe drought that occurred between 1924-1939 caused a decrease in mean annual discharges, allowing the establishment of vegetation along banks and in locations that normally would have been the mean high water level. At the same time irrigation allowed for perennial flow and sufficient moisture to sustain dense vegetative growth. The historical survey data clearly demonstrat~s a trend for channel narrowing. This trend has been documented both upstream and downstream from John Martin Dam. As suggested by Nadler (1978) increasing vegetation along the banksiis probably the most significant factor causing the narrowing. A field reconnaissance of the Arkansas River was conducted 30 April through 1 May 1997. The river channel was observed at several locations upstream and downstream from John Martin Dam. River widths were observed to be highly variable. Based on limited site observations it appeared that generally, existing river widths upstream from John Martin Reservoir were greater than downstream. In most o~served cases, the banks were heavily vegetated. Sites where vegetation had been washed put had active bank erosion and were wider. During the field reconnaissance bed-material samples were collected at five sites on the Arkansas River and from two tributaries. The river bed was found to consist primarily of medium to course sand. Bed-material gradations ar~ shown in Figure 3. Two samples were collected upstream from John Martin Reservoir-on~ at Bent's Old Fort Historical Site and one at the Highway 207 bridge, which is located about 30 miles further upstream. These samples indicated a median grain size of about 0.45 rnm. A geomorphological assessment of the Arkansas River below John Martin Dam was completed in early 1997 by the U.S. Army Corps of Engineers Waterways Experiment Station. The assessment provides an overview of the geologic, climatologic, and hydrologic forces influencing the Arkansas River during the past century. The assessment includes information pertaining to above John Martin Reservoir, as well, which was used in this study. For more detailed information of this assessment refer to: "19~9 Arkansas River From John Martin Dam to the Colorado-Kansas State Line: Channel Capacity and Riparian Habitat Planning Study, Appendix C. 3.2 HYDRAULICS AND SEDIMENTATION 3.2.1 Historic Hydrology The Colorado Water Conservation Board requested assistance from the Albuquerque District, to delineate the discharge frequencies along approximately 80 miles of the Arkansas River from Pueblo Dam downstream to John MartinlReservoir. These discharge frequencies will be used in future studies and hopefully assist in developing land use planning and stream corridor management studies.