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Cross Section Surveys: Detailed measurements of channel properties were taken in a 1- <br />km long reach centered around RKM 283 (RM 176) to provide more detail on channel changes <br />and to model thresholds for bed load transport. This particular segment of the Colorado River <br />was chosen because conditions within the reach are relatively natural; the reach includes a <br />through-flowing secondary channel, alluvial channel margins with a limited amount of rip-rap, <br />and well-defined floodplains along both the north and south sides of the channel (Fig. 5). In <br />addition the study reach includes property on the south bank that was obtained by the USFWS <br />and the Bureau of Reclamation, and is therefore relatively easy to access. <br />Figure 5. Location of reach used for detailed studies of channel change. <br />Initial topographic surveys of the study reach were conducted in May, 1998. Eleven <br />cross sections were placed at evenly spaced (-80 m) intervals through the reach, covering a total <br />channel length of 800 in. Measurements of the channel-bed and water-surface elevations were <br />taken with a total station and a rubber raft outfitted with a depth sounder. Survey measurements <br />of the cross sections were repeated in August, 1998; October, 1999; and July, 2001. Separate <br />measurements of the water surface elevations were taken periodically throughout the study for <br />14