North Poudre Sediment Study Prog Report

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In addition to the repeated sediment transport sampling, we collected random grab samples from pool margins throughout the study reach in Oct. 1996; these were wet-sieved and weighed. Coarse sediments forming the channel bed in rifiles and runs were measured during random-walk clast counts. Infiltration of fine sediments into gravel and cobbles at the tail of pools was measured with a surface and a subsurface grab sample from the upstream pool in Reach II during Oct. 1996. The infiltration sites will be re-sampled as soon as flow is reduced to a minimum for the 1997-98 winter. Results 1) Initial depositional patterns, The majority of the initial sediment deposition occurred in pools, Pools immediately downstream from the dam were completely filled, with more than 3m of vertical deposition in the pool thalweg, Adjacent riftles and runs had relatively little deposition. Small bars (up to 2m long and 1m wide) formed along the channel margins in areas offlow separation along rifiles and runs, and fine sediment infiltrated the matrix of the channel bed to a depth of 6 cm, Rooted aquatic plants growing from the channel bed along the rifiles were still present and unburied following the &ediment release, suggesting that the sediment-release flow was not very erosive. Sediment deposition followed a distinct fining trend downstream. Sediment filling pools immediately below the dam had a substantial component of sand and fine graveL The sediment filling pools in study Reach II was primarily clay to fine sand (Figure 2a), Sediment concentration also decreased downstream from the dam. At Reach I (500 m downstream), the pool was completely filled, At Reach II, pools 2,8 and 3.2 km downstream lost approximately 65% and 50% of the volume to sediment infill, respectively, At Reach III, a pool 4.9 km downstream lost 30% of its volume, Sediment deposited in the pools of Reaches II and III formed a veneer of fairly uniform thickness over the pool bottom, suggesting that it settled from suspension. High- water marks from the sediment-release flow correspond to a maximum discharge of 140 m3/s. 2) Repeat surveys, The pools in Reaches I and II were re-surveyed on 31 March 1997, following a month of flows that rose to 3,93 m3/s (Figure 3), These surveys revealed two distinct trends, (1) Pool geometry had been altered as a result of sediment erosion and deposition, Each of the pools has a relatively narrow (14, 19, and 22 m, respectively) upstream width where flow enters the pool from a steep rifile. At approximately half of the pool length, width lessens considerably (10, 16, and 12 m) in association with a bedrock constriction along one wall, and maximum depth occurs immediately downstream (1-2 m) from this constriction. Width again increases (20, 21, and 26 m) at the downstream end of the pool, where the bed rises to a maximum elevation before dropping off steeply into the next rifile, As noted previously, the initial sediment release had largely created an even-depth veneer of fine sediment over this topography at the pools in Reaches II and III Subsequent reworking created a steep-sided, narrow thalweg at the upstream end of each pooL Sediment was scoured to the pre-sediment-release bed in this thalweg, but , additional sediment was deposited along the channel margins on top of the Sep, 1996 sediment (Figure 4a). The pool at the constriction had relatively little net change. The thalweg scoured to nearly the original bed Ieve, and shifted laterally, but some additional sediment was added to the channel margin opposite the bedrock constriction (Figure 4b), In each case, the downstream