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Green River Geomorphic Responses

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9722-C63 - Geomorphic Responses of the Middle Green River to Floods - Completed FY2008

Problem - Water, sediment, and vegetation are the primary elements controlling the form of large river channels in the Pacific Northwest and comprise the physical template for river ecosystems. Dams interrupt the flow of all of these elements, with profound consequences for the spatial distribution of aquatic and riparian habitats and the populations of species using these habitats. An understanding of how streamflow, sediment, and vegetation interact to form and maintain habitats is the first step in assessing the ecological effects of dams and developing hypotheses about how these effects can be managed effectively.

The middle Green River in King County, Washington, presents a challenging situation for water-resources managers who must address both ecological needs for a natural streamflow regime and social needs to prevent flooding. The river supports populations of threatened chinook salmon, but flood flows have been limited since 1962 to a peak magnitude of 12,000 cfs (about equivalent to the pre-dam, 2-year flood) at USGS Station 12113000 by upstream regulation at Howard Hansen dam to protect residential and commercial developments in the floodplain. To address these competing needs, a group of scientists and water-resources managers in the Green River basin has developed the Middle Green River Flow Investigation (MGRFI) plan. The proposed investigation focuses on the problem of quantifying the geomorphic responses of the Green River to floods and represents an initial step toward implementing the MGRFI plan.

Objectives - The objective of the proposed investigation is to develop a method for quantifying the geomorphic responses of an alluvial river to floods. The investigation will focus on geomorphic responses that are detectable using aerial photography and that can be linked to ecological processes and conditions. The method will be demonstrated in selected reaches of the Green River and will provide a prototype in future studies for a geomorphic analysis of the entire middle Green River from Howard Hansen dam to Big Soos Creek.

Relevance and Benefits - This investigation initiates research efforts planned under the MGRFI, which represents the research priorities of a broad-based group of local, state, Federal, and tribal water-resources managers in the Green River basin. The results of the investigation will contribute to the scientific basis for ongoing management of the river ecosystem. It is consistent with the long-term program goal of the USGS to "provide and improve long-term environmental and natural resources information, systematic analyses and investigations, and predictive tools for scenario building and decision making about natural systems" (USGS Strategic Plan, May 12, 2000, version).

Approach - The investigation has four primary tasks: 1) develop a conceptual model of geomorphic responses of an alluvial river to floods; 2) identify metrics for measuring geomorphic responses from aerial photographs; 3) digitize geomorphic features in a time-series of aerial photographs of three selected reaches (~1 km each) of the Green River and calculate the selected metrics; and 4) prepare a report documenting the methods and results of the geomorphic assessment.

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