River Hydraulics and Sediment Transport

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?1. L;UMMARY OF IMPACTS Each alternative has a land component and a water component that influences the river channel habitat. Improvements to habitat in the short term are most directly the result of the mechanical activities of the land plan for each alternative. However, over the long term, the success of the alternative in improving channel habitat becomes more strongly influenced by the water plan for each alternative. Alternatives with land plans that include the clearing and leveling of river islands would reduce or eliminate the trend of river channel degradation that is presently occurring downstream from the J-2 Return Channel (see the "River Hydraulics and Sediment Transport" section in chapter 4). The clearing and leveling of river islands would locally create a wider open view width for endangered birds and provide more sand, and at a finer grain size, than is presently available to the river. Water plans with the release of short- duration, high flows near bankfull capacity would help sustain the open view widths at the managed sites by scouring seedling plants that begin to grow along the river channel bed and banks. The short-duration, high flows near bankfull capacity, along with the additional supply of finer grain sand, would help to create more sandbars and at higher elevations than under the Present Condition. All of the alternatives undertake measures which can affect the characteristics of the river channel in the Central Platte Habitat Area and the quality of the habitat provided there for the three target bird species. This occurs primarily through the changes in riverflow, described in the previous section, coupled with direct management of channel width and sediment load in the river, described in detail below. Ultimately, these actions affect the amount of usable roosting, nesting, and foraging habitat available in the river channel for the target bird species. ALTERNATIVES COMPARISON FOR FLOW, SAND TRANSPORT, AND GRAIN SIZE The water plans for the Water Leasing and Water Emphasis Alternatives have the greatest mean flows with differences of 4 to 8 percent more flow than the Present Condition. The 1.5-year flow values indicate that Governance Committee Alternative, Scenario l, does not have a significant pulse flow in comparison to the remaining alternatives, while the Wet Meadow Alternative has marginally smaller peaks than the Governance Committee, Scenario 2; Water Leasing; or Water Emphasis Alternatives.