North Poudre Sediment Study Prog Report

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~ portion of the pool aggraded substantially (up to 1m), As a result, pool geometry effectively became a deep, narrow thalweg bordered by high sediment bars or bedrock walls for the upper two-thirds of the pool, and a much broader and shallower cross section in the downstream portion where the central jet of relatively high-velocity flow dissipated in rising over the pool tail. The September 1997 resurveys indicated that the central thalweg of the pool had been widened and deepened, and the pool tail had scoured to the original bed, leaving only fairly narrow (2m wide) but high marginal bars for much of the pClollength (Figure 4). These marginal bars are now becoming vegetated, and have the potential to become long-term channel boundaries, (2) Grab samples of sediment along the pool margins indicated that sediments deposited along pools in Reach IT during the February-March 1997 flows were coarser than those from the Sep, 1996 release. In contrast to the predominantly clay to fine sand deposited in Sep, 1996, the March 1997 surveys indicated lateral deposits Clfmedium to coarse sand immediately adjacent to the thalweg, with deposits of clay to fine sand along the channel margins. Essentially, the sediments present at the pool in Reach I grew finer with time (Figure 2b), whereas those at downstream pools grew coarser with time (Figure 2c), The Oct. 1996 measurement indicated that fine sediment infiltration into the gravel and cobbles at the pool tails reached a maximum depth of 6 cm below the surface. We will repeat these measurements when flow declines for the year. (3) Suspended sediment sampling. Suspended sediment was collected through 21 May 1997, After this date suspended sediment transport Was too low to be measured, despite the fact that flow did not peak until 3 June 1997, Suspended sediment transport along the North Fork Poudre River showed a modified version of the hysteresis effect described for many other rivers, The bulk of suspended sediment transport occurred during the rising limb of the initial flow pulse (22 Feb,-30 March 1997), When discharge began to rise again on IS April, suspended sediment transport remained negligible until discharge reached three times the earlier peak (Figure 3), but transport rates remained lower than the preliminary peak, The downstream sampling cross section had consistently higher suspended sediment concentrations than the upstream cross section, (4) Bedload data, The four bedload sampling points were used to estimate a total sediment discharge for each cross section on each sampling day, We assumed that sediment discharge (gls) for each sample point represented an area of channel bed I m wide centered on the sample point, and calculated a weighted average of total sediment discharge for the cross section, Using each sample point as representative of only the 7.6-cm-wide area covered by the Helley-Smith sampler, we also calculated an average unit sediment discharge (gls) for the four sampling points on each sampling day, Finally, we calculated grain-size parameters (d,o, d9,) for a composite ofthe four samples at each cross section on each sampling day. (d9, is grain size for which 95% of the sediment in a sample is smaller,) , The two sampling cross sections showed distinctly different trends of bedload movement, The upstream cross section had no correlations between volume of sediment transport and hydraulic