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In publishing suspended sediment data, the USGS also often reports the percentage of sediment finer than 0.063 mm (the sand-silt break), and sometimes reports the full particle size distribution of the suspended sediment. These data are useful for evaluating long-term trends in sediment load because the sediment concentration usually increases as the percentage of sand increases. The measurements from the Cisco gauge are by far the most complete in this regard; this data set includes sand-fraction analyses for nearly 300 samples, dating back to 1951. Some sediment-size analyses are available for measurements at the Cameo and State Line gauges, but we have not evaluated these data in detail because they are all from the more recent time period (1979-present). Field Studies Field studies were conducted from 1995 through 1998 to characterize the geomorphology of the study reach, and to obtain the data necessary to model sediment transport in individual subreaches. Cross Section Surveys: Cross sections of the main channel were surveyed at evenly spaced, 1.6-km intervals from RKM 366 to RKM 77, with the exception of the two reaches that were difficult to access, and a few sites that could not be surveyed for logistical reasons. Cross sections were surveyed using an electronic theodolite (total station) and a motorized rubber raft outfitted with a depth sounder. To survey a cross section, the total station was set up over one of the endpoints. Distance and depth readings were then taken along the line of the section by targeting a prism on the raft, while at the same time, the person operating the raft would read the depth off the chart recorder and relay the data by radio to the person on shore. Bed Material Samples: Point counts of the surface bed material (pavement or armor layer), and bulk samples of the subsurface sediment were used to characterize the bed materials throughout the study reach. Samples were taken from exposed gravel bars at low flow (in most places, the water was too deep, even at base flow, to allow us to sample across the entire bed, therefore, we could not sample consistently from the same morphologic features, e.g. riffles). In our experience, textural variations from bar to bar are often relatively large, thus for purposes of sediment transport calculations, we grouped the data from individual sites to obtain composite samples for the various subreaches. The average values of D50 listed in Table 1 are, therefore, based on anywhere from 3 to 10 samples in a reach. Surface sediment samples were taken by randomly selecting 100 or 200 particles (the so-called Wolman method) and determining their sizes using a metal template with openings equal to standard sieve sizes. The subsurface samples were taken by first removing the surface layer of particles, then collecting 100-150 kg of the sediment underneath. The coarse fraction of this sample was sieved in the field, and the fine fraction was sieved in the laboratory. A total of 56 surface samples and 27 subsurface samples were taken in the 300-km study reach. Bed-Material Transport Thresholds: Estimates of discharges required to transport the bed material of the Colorado River were made by combining several conventional flow and sediment transport equations, solved and calibrated with the aid of the field data described above. The most important and fundamental equation in this analysis is the Shields' parameter: T (4) (R- P)gD where f is the dimensionless shear stress, r is the near-bed shear stress, ps and p are the densities of sediment and water, respectively, g is the gravitational acceleration, and D is the particle size. Equation 4 is essentially a balance between the fluid force causing motion and the weight force (or friction) resisting motion. The variable f is continuous, but discrete values defined on the basis of 16