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grain size varies from place to place. Fortunately, during the most recent drought year <br />streamflows in the 15-mile reach dropped to the point where it was possible to wade the channel <br />and sample the bed surface in all but the very deepest parts of the channel (fine sediment <br />covering higher surfaces on subaerially exposed bars was ignored). These measurements <br />indicate that the bed sediment is generally coarser in the thalweg than it is on the bars, as <br />expected. However, the difference in grain size is not very large, except in the sections spanning <br />the riffle (sections 5-6). The riffle includes many boulders and large cobbles, leading to a <br />coarse-tailed grain size distribution (D50 -100 mm). In addition there is an short segment of <br />channel between sections 5 and 6 that is floored by bedrock. The presence of bedrock and <br />coarser-than-average sediment within this part of the study reach is indicative of locally high <br />shear stresses produced at certain flows. <br />Table 5. Comparison of bed surface samples taken from exposed bars and deeper parts of <br />the channel, RM 176. Samples at cross sections 1-5 were taken only across the <br />submerged portion of the channel; samples at the other cross sections were taken <br />across exposed bar surfaces and across deeper parts of the channel. <br /> morphology median grain size, median grain size, <br /> exposed bar (mm) thalweg (mm) <br />XSECT 1 run 60 <br />XSECT 2 pool -- <br />XSECT 3 pool 52 <br />XSECT 4 pool 59 <br />XSECT 5 riffle 99 <br />XSECT 6 riffle 62 80 <br />XSECT 7 run 80 84 <br />XSECT 8 run 76 76 <br />XSECT 9 run -- -- <br />XSECT 10 run 74 61 <br />XSECT 11 run -- -- <br />53