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<br />If t*c is 0.03 (Neill, 1968), then for the average water-surface slope <br /> <br />(5=0.00069), and the minimum observed mean depth (0=80, see table 7 in the <br /> <br />Supplemental Data section at the end of this report), d =7.69 mm. Sediment <br />c <br />particles of this size on the bed are at the threshold 'of movement, and all <br /> <br />smaller material is mobile under these flow conditions. The median grain size <br /> <br />(dso) of bed material in the study reach (table 2) is considerably smaller <br /> <br />than dc (7.69 mm) for this flow depth. The average size of bed material at <br /> <br />the 95th percentile (dgs=3.13 mm) also is smaller than dc, indicating that the <br /> <br />majority of material composing the streambed at Deerlodge Park is a size <br /> <br />capable of transport at the lowest flows. <br /> <br />No obvious nonlinear trends were detected in the sediment discharge and <br />water discharge data; therefore, sediment transport equations describing the <br />variation in measured sediment discharge as a function of the water discharge <br />were derived by a least-squares 1 i near regression of the log-transformed <br />values. Equations were determined separately for total sediment discharge, <br />suspended-sediment discharge, bedload discharge, and for different size <br />fractions of sediment: coarser than 0.062 mm (sand and gravel), finer than <br />0.062 mm (silt and clay), 0.062 to 0.25 mm, 0.25 to 1.0 mm, and coarser than <br />1.0 mm (table 3). Based on values of the coefficient of determination (R2), <br />and standard error of estimate (SE), linear-regression analysis was adequate <br />to account for most of the variance in data of four of the eight sediment <br />categories: total sediment discharge, suspended-sediment discharge, discharge <br />of material coarser than 0.062 mm (sand and gravel), and discharge of material <br />0.062 to 0.25 mm (fine sand). Variance of the data in other sediment catego- <br />ries)'las poorly accounted for by linear-regression analysis. <br /> <br />The regression equation for silt and clay-size material (finer than 0.062 <br />mm) in table 3 did not predict well the discharge of this material. The <br />proportion of silt and clay in the suspended-sediment discharge varied consid- <br />erably (see table 8 in the Supplemental Data section at the end of this <br />report). While the amount of sand transported in suspension varies in phase <br />with discharge, the amount of silt and clay transported is controlled by the <br />supply of these fine materials to the stream. The supply of fine material to <br />a stream reach is influenced by seasonality, the size and duration of runoff <br />generating storms, and the lag effect of the downstream travel of sediment and <br />water waves (Richards, 1982). Regression equations for bedload discharge, for <br />di scharge of material in the 0.25 to 1. 0 mm range, and for materi a 1 coarser <br />than 1.0 mm also did not predict well. Transport of coarse sand and gravel is <br />strongly di scharge dependent. Vari ance in these data is 1 arge because the <br />sediment discharges of these size ranges were derived from measured bedload <br />discharges, which also had large variances. The variance in measured bedload <br />di scharge may be 1 arge due to a number of reasons, i nc 1 udi ng temporal vari- <br />ability in bedload discharge which was not accounted for in the sampling <br />procedures (D. W. Hubbell, U. S. Geological Survey, oral commun., 1984). <br /> <br />Seasonal differences in the supply of sediment to a stream or in water <br />temperature may affect sediment discharge. Some rivers, for example, the San <br /> <br /> <br />15 <br /> <br />