North Poudre Sediment Study Prog Report

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. - parameters (shear stress and discharge), or between grain-size parameters and either hydraulics or flow duration (days since the start of sampling), In contrast, the downstream sampling cross section had positive correlations between volume of sediment transport and hydraulic variables, and between grain-size parameters and hydraulic variables, The bedload grain-size distribution also showed a dramatic change between 13 and 16 May 1997, during the rising limb of the principal spring flow peak (Figure 5), In general, both total and average sediment discharge were lower at the downstream cross section than at the upstream until flow began to rise dramatically on 13 May (Figure 3), Bedload at the downstream sampling cross section was consistently finer- grained, although the values of ct., became approximately equal at both cross sections after 16 May 1997, Discussion Much of the fine sediment deposited in pools along the North Fork Poudre River below Halligan Reservoir was eroded and transported downstream during the spring-summer high flows of 1997. This occurred in part because the 1997 high flow was greater than an average year. The specific sedimentation patterns present in each pool were a function of distance downstream from the primary sediment source (the reservoir), and of discharge magnitude and duration since the initial sediment release (as these governed travel time of sediment between pools), Sedimentation was also a function of pool geometry, and of sediment transport and storage along the riffle immediately upstream from each pool. The lateral constriction and downstream expansion present in pools along the North Fork Poudre River create a central jet of relatively high velocity flow with associated flow separation and eddy circulation along the pool margins, This jet dissipates at the downstream end of the pool, where the reverse bed-gradient of the pool exit- slope also promotes sediment deposition, During high flows this central jet scours the pool thalweg, but does not prevent substantial deposition along the lateral eddies. As discharge decreases and the central jet velocity declines, some of this lateral sediment may slump back into the pool thalweg and be re-entrained, Storage of sediment along riffles may also partly control downstream bedload movement. Initial deposition along the downstream riffles took the form of relatively small, isolated marginal deposits of clay to fine sand, These deposits were exposed above the low-flow water level during the winter, and hardened to the consistency of weak concrete, Interparticle cohesion was so high that many of these deposits remained substantially unchanged throughout the 1996-97 flow season. Sand and gravel were not initially observed along the downstream sampling cross section, but appeared among the cobbles on the bed between approximately 13 and 16 May 1997, and persisted throughout the sampling period. In contrast, the thalweg of the upstream sampling cross section was initially covered with sand and gravel a few centimeters thick. Within ten days this veneer had been removed except along the channel margins or in the lee of protruding boulders, where it formed sand ribbons, The sediment stored in these isolated deposits gradually became coarser grained as the deposits grew smaller and were eventually largely removed, The differences in bedload transport between the upstream and downstream sampling cross sections presumably reflect these controls. The upstream sampling cross section has only one upstream pool, and began with an initially coarser mobile-sediment distribution (i.e., a veneer of