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that did move, consistent with the expectation that small fractions of the bed surface are <br />mobilized by flows lower than the reach-average threshold for initial motion (-50% of the <br />bankfull discharge). This discharge falls well short of a "channel maintenance flow", but is <br />nonetheless capable of mobilizing a handfull of rocks within an area of a few square meters. <br />The most vivid illustration of discharge-related changes in channel properties within this <br />segment of the Colorado River is the dramatic growth in vegetation on low-lying bar surfaces. <br />Figure 29 compares downstream views of the Colorado River, taken four years apart at the same <br />location on the lateral bar at RM 176. This location was essentially devoid of vegetation in 2000 <br />but is covered with waist-high tamarisk by 2004. The bar surface show in these photographs was <br />Figure 29. View downstream showing growth of vegetation on the lateral bar, RM 176. <br />56