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QUALITATIVE MEASUREMENTS AND OBSERVATIONS <br />The analysis and results discussed in the preceding sections provide quantitative <br />information on the geomorphic effects of managed and naturally occurring streamflows in the <br />15-mile reach of the Colorado River. The field surveys and modeling results generally support <br />the recommendations from previous studies. Further evidence of the geomorphic effects of flow <br />levels is illustrated below with ground-based photographs taken at various times for different <br />purposes. The first set of photographs (Fig. 28) shows results from an experiment in 2001 that <br />was used to assess the extent of bed material entrainment within small areas (patches) of the bed. <br />Rectangular patches of the bed surface were washed using a portable water pump and cordless <br />drill; the surface was then allowed to dry, and spray painted brightly colored paint. Each patch <br />was located with the total station and photographed. The patches were relocated after the peak <br />in snowmelt runoff and photographed again. <br />r <br />Figure 28. Before- and after-photographs of painted rocks at cross section 7, RM 176. <br />The patch shown above was submerged under -1 in of water during the peak discharge in <br />2001 (227 m3/s). The shear stress in the vicinity of the patch under these conditions would be <br />about 20 N/mZ, which is 30% less than the threshold for motion. The photographs show that the <br />majority of rocks within the patch did not move; however, it is possible to identify several rocks <br />55