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Messrs. Lochhead, Simpson, and Robotham 10 <br />Comment: "How will pulse flows affect channel maintenance when much of the <br />system's historical sediment load is trapped behind large mainstream dams ?" <br />Experts questioned at the May 1994 pulse flow workshop were uncertain of how <br />the system would respond and, therefore, they recommended that gradual phasing <br />with monitoring was essential to pulse flow management. They also briefly <br />----!> discussed liberation of sediment being trapped and accreted in narrow channel <br />reaches. The availability of sediment supply and movement is important to <br />channel morphology, but not all aspects of sediment budget are known. <br />----),Augmentation of flows from the North Platte River should consider the incoming <br />bed material load and flow of the South Platte River. Development of the <br />lower South Platte River that curtails sediment movement would be a <br />consideration. Structural and operational measures such as those recommended <br />by the Department to FERC for the Korty Diversion Dam (FERC Project 1835) and <br />the Tri- County Diversion Dam (FERC Project 1417) would avoid intake of bed <br />material load into the respective canals and facilitate movement of bed <br />material load past the structures. <br />Comment: "Have mechanical approaches to channel maintenance been considered <br />as an alternative to pulse flows? If so, what are the results ?" <br />Mechanical clearing is practiced in some limited areas in attempts to restore <br />horizontal visibility for cranes by knocking down vegetative growth in the <br />former river channel. This is done in anticipation that high flows would <br />scour and help maintain the channel. Thus far, the results are large piles of <br />dead trees and large banks of disced sand and gravel. Mechanical clearing <br />does not accomplish a variety of functions of pulse flows. <br />Comment: "Have man - induced encroachments on channel width been addressed in <br />determining and assessing possible benefits of the recommended target flows ?" <br />Dr. Bob Simons (Simons and Associates 1990) indicated that bridges earlier had <br />a local effect on channel morphology. Other encroachments on channel width <br />(e.g., construction of riverside houses, hunting cabins, private bridges, and <br />agriculture on accreted river channel) appear to increase with reduced peak <br />flows. <br />Page 7. 3., Forage Fish <br />For purposes of clarification, seasonal base flow targets are based on a <br />number of purposes including physical microhabitat for the fish community, <br />aquatic community, and (for summer periods) least tern and piping plover <br />nesting. Therefore, they are based on a balance of information regarding <br />physical microhabitat modeling for fish, hydraulic simulation (wetted area), <br />water temperature studies and observed fish die -offs, and professional <br />opinion. <br />Hydraulic simulations for the central Platte River (Figure 1) show that the <br />decline in wetted area accelerates at flows reduced below about 1,100 to <br />1,300 cfs. The rate of decline is most rapid below about 600 to 800 cfs. As <br />water recedes from margins of the channel where important fishery habitat, <br />organic material, root wads, banklines, and rooted annual vegetation becomes <br />