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I I I I I I I I I I I 1799 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 resisuj 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 vegetati ve growth. The historical survey data clearly demonstrates a trend for channel narrowing. This trend has been documented both upstream and downstream from John Martin Dam. A1J suggested by Nadler (1978) increasing vegetation along the banks:is 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 rivef widths upstream from John Martin Reservoir were greater than downstreBm. In most o~served cases, the banks were heavily vegetsted. Sites where vegetation had been washed put had active bank erosion and were wider. During the field reconnaissance bed-material samples were coIlected 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 are shown in Figure 3. Two samples were collected upstream from John Martin Reservoir-one 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 mrn. 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. '1Vhich was used in this study. For more detailed information of tbiB 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 req'uested assistance from the Albuquerque District, to delineate the discharge frequencies along BpproximBtely 80 miles of the Arkansas River from Pueblo Dam downstream to John Martin'Reservoir. These discharge frequencies will be used in future studies and hopefully assist in developing land use planning and stream conidor management studies. 24 3 I I I I I I' I I I I I I I I I I I I I