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<br />2005 <br />downstream ranges indicated channel narrowing due to aggradation through 1987 with <br />main channel degradation at the most downstream range and main channel aggradation <br />at the other. <br /> <br />9. The historical survey data clearly demonstrates a trend for channel narrowing. This <br />trend has been documented both upstream and downstream from John Martin Dam. Therefore, <br />narrowing of the river channel between John Martin Dam and the Kansas state line cannot be <br />totally attributed to construction of the flood control reservoir. As suggested by Nadler (1978) <br />inCreasing vegetation along the banks is probably the most significant factor causing the <br />narrowing. However, the survey ranges downstream from the dam indicate a continuing <br />degradation and narrowing trend after construction the dam through 1987. This trend was not <br />documented in the Nadler study. It is recommended that the ranges reported by Nadler be <br />resurveyed to assess changes through 1997. Further it is recommended that the 13 ranges <br />downstream from John Martin Dam be resurveyed to document recent changes in this reach. <br /> <br />Field Reconnaissance <br /> <br />10. A field reconnaissance of the Arkansas River was conducted 30 April through 1 <br />May 1997. The river channel was observed at several locations upstream and downstream <br />from John Martin Dam. River widths were observed to be highly variable. Based on limited <br />site observations it appealed that generally, existing river widths upstream from John Martin <br />Reservoir were greater than downstream. In most observed cases, the banks were heavily <br />vegetated. Sites where vegetation had been washed out had active bank erosion and were <br />wider. Bank erosion of unprotected banks was observed both upstream and downstream from <br />the dam, indicating that even with the dam in place, it is possible to attain discharges and <br />velocities sufficient to wash out bank vegetation. Figures 3 and 4 show contrasting channel <br />widths at two locations downstream from John Martin Dam. Longitudinal frequency of bank <br />failures upstream and downstream from the dam can be documented by more extensive field <br />work or by inspection of aerial photography. <br /> <br />11. During the field reconnaissance bed-material samples were collected at five sites <br />on the Arkansas River and from two tributaries. The river oed was found to consist primarily <br />of medium to coarse sand. Bed-material gradations are shown in Figure 5. Two samples were <br />collected upstream from John Martin Reservoir -- one at Brent's Old Fort Historical Site and <br />one at the Highway 207 bridge, which is located about 30 miles further upstream. These <br />samples indicated a median grain size of about 0.45 mm. Samples collected downstream from <br />the dam were coarser. The median grain sizes decreased in a downstream direction. Down- <br /> <br />Arkansas River, Colorado <br />Geomorphological Assessment <br /> <br />7 <br /> <br />August 8, 1997 <br />