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<br />. <br /> <br />. <br /> <br />Main Channel Cross Sections <br /> <br />Cross sections of the main channel were surveyed every 5 miles, extending from RM 185 <br />downstream to RM 130 (Fig. 1). Survey measurements in the IS-mile reach indicate relatively <br />l minor changes at cross sections RM 185 and RM 175 in 1995, but what appear to be much more <br />I <br />significant changes at RM 180 (Fig. 10). We are not certain, however, that the changes at RM 180 <br /> <br />. <br />1 <br /> <br />are 'real' because during the 1995 survey we were unable to find either one of the cross section <br />endpoints, and thus it was not possible to re-Iocate the section line exactly (at least one, if not both, <br /> <br />of the endpoints for all other sections were located, and thus these surveys are accurate within the <br /> <br />limits of the techniques). In any case, the most recent surveys of RM 185 and RM 175 indicate <br /> <br />localized scour and fill of up to 0.5 m in the channel (Fig. 10), but little change overall. <br /> <br />Measurements of cross sections in the 18-mile reach indicate similar trends: localized scour <br /> <br />and fill, but little change in morphology overall (Fig. 11). Several meters of the right bank were <br /> <br />eroded at RM 170, and about 0.5 m of sediment was eroded off the top of the bar at RM 155. <br /> <br />Otherwise, the changes observed at cross sections in this reach were minor. <br /> <br />Cross sections in Horsethief Canyon likewise exhibited localized scour and fill of up to 2 m <br />in the main channel, but generally much smaller changes (Fig. 12). No appreciable changes in <br /> <br />channel morphology occurred in this reach, largely because the river is more constrained here by <br /> <br />bedrock. The remaining two cross sections, RM 135 in Ruby Canyon and RM 130 in the alluvial <br />reach upstream of the Westwater boat launch, exhibited contrasting responses (Fig. 13). The cross <br />section at RM 135 showed no discernible change, whereas the cross section at RM 130 showed the <br />most significant change of any that we measured. More than 30 m of the left bank was eroded at <br />RM 130, taking out the original left endpoint of the section (the right endpoint was located and the <br />section line re-established on the basis of ground photographs). The changes at RM 130 appear to <br />be the direct result of channel instability in the area a few hundred meters upstream. This area is <br /> <br />characterized by a number of channels and bars that continue shifting location. It also includes a <br /> <br />site where spawning activity has been suspected in the past (D. Osmundson, pers. corrun.), and <br /> <br /> <br />thus it may be important to continue monitoring channel change in this area. <br /> <br />13 <br />