Laserfiche WebLink
4.0 Summary and Conclusions <br />CDM has completed Task 34 by performing the following activities at 30 test sites <br />along the South Platte River mainstem and some of its tributaries. <br />• Collected 175 streambed sediment samples from the 30 sites and had these <br />analyzed for grain-size distribution; estimated K based on median grain-size. <br />• Performed hydraulic testing of the streambed materials, resulting in K values <br />that were compared to estimates based on grain-size distribution. <br />• Surveyed stream channel cross sections and determined their elevations. <br />• Determined the vertical gradient between the stream and fl1e underlying <br />aquifer for t11e discrete time of measurement. <br />Below are conclusions from completion of these tasks. These findings will be used in <br />developuzg iizputs to the stream package uz the alluvial model beuzg developed under <br />Task 48. <br />streambed sediments along fl1e mainstem of the South Platte River and in the <br />Cache la Poudre River are poorly sorted and predominantly in the medium to <br />coarse-grained size range. In contrast, streambed sediments >11 tributaries <br />overlying the Denver Bas>11 tend to have f>lzer-grained materials, which have a <br />mean grails-size about 11a1f t11at of the mainstem. T11e finer grain-size of <br />streambed sediments in the tributaries overlying the Denver Basin is likely <br />due to a combination of factors including finer-grained source materials of the <br />bedrock deposits, lower streamflows and lower stream channel gradients that <br />exist in this area compared to fl1e mountainous drainage area and much <br />higher flow of the South Platte and Cache la Poudre Rivers. <br />• The similarity of sediments throughout fl1e entire cored interval at all sites <br />indicates that no distinctive streambed is present to a depth of 3 ft below the <br />top of the streambed at the locations evaluated. <br />The South Platte River streambed is dominated by riffle and pool morphology <br />upstream of Greeley and by braided and riffle and pool morphology <br />downstream of Greeley. Tributaries exhibit a variety of morphologies <br />including channelized, incised, and riffle and pool. This channel morphology <br />will be used in the Task 48 modeling to define the stream channel geometry of <br />all simulated streams. <br />The active portion of the South Platte River channel was observed to have <br />widths of 50 to 100 ft between Chatfield Reservoir and Greeley, The South <br />Platte River widths widen to 150 to 200 ft between Greeley and Fort Morgan, <br />and flley narrows to as little as 30 ft near Julesburg. This channel width <br />information will be used in the Task 48 modeling to define the stream channel <br />geometry of all simulated streams. <br />Fnlal T~zsl: 3~TM 06-28-07.doc 13 <br />