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Summary report to assist development of ecosystem flow recommendations for the Coast Fork and Middle Fork of the Willamette River, Oregon

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  • Introduction The Willamette River Flow Project The Nature Conservancy (TNC) and the U. S. Army Corps of Engineers (USACE) are collaborating on a project to determine environmental flow requirements for the Willamette River and its tributaries and to design and test alternative flow releases from the dams that can meet these requirements. The project is part of the Sustainable Rivers Project (SRP), a national effort by TNC and USACE to investigate opportunities to change Corps Dam operations (“reoperate”) to achieve more ecologically sustainable flows, while maintaining or enhancing project benefits. Through the SRP, TNC and USACE have developed and tested a process for identifying and refining environmental flow objectives (Richter et. al. 2006). The process utilizes a series of steps to define environmental flow requirements, implement changes in operation of dams to meet those flow objectives, monitor and model the effects of those changes on both the river ecosystem and the operation of the dams, and refine over time. The Willamette River Flow Project is being conducted in conjunction with the USACE Willamette Floodplain Restoration Feasibility Study. This feasibility study is designed to identify opportunities to restore natural floodplain function in the Willamette River basin to provide ecosystem restoration, natural flood storage, and other benefits. The initial study phase has focused on the Coast and Middle Forks of the Willamette River. These subbasins contain 6 of the 13 dams in the Willamette system; their operation has implications for the operation of the other dams in the system. To date, the Floodplain Restoration Study has focused on two important aspects of the aquatic ecosystem: 1) identifying habitat, flow and water quality requirements for a variety of aquatic and floodplain species; and 2) describing and evaluating the current channel and floodplain morphological characteristics, and their changes from historic condition. Partners in the feasibility study include the Willamette Partnership, the Willamette Middle and Coast Fork Watershed councils, the Oregon Department of Fish and Wildlife, and The Nature Conservancy. There is also an inter-disciplinary expert and stakeholder group of approximately 20 federal, state and local entities plus private landowners that informs the process. The Willamette River Flow Project will build upon the Floodplain Restoration study by developing environmental flow requirements for the reaches downstream of the Corps dams and linking those flows to opportunities for stream channel and floodplain restoration, and to improvement in operation of the dams. Given the existing floodplain restoration study, the initial SRP efforts will use the Coast and Middle Forks and the mainstem Willamette immediately downstream of these tributaries as a pilot study that can be replicated in the rest of the Willamette system. River flows from both subbasins have been greatly affected by operation of the dams: 56% of the drainage area of the Coast Fork and 87% of the Middle Fork drain into USACE reservoirs. General effects of the reservoirs include reduced peak flows, lower spring flows, increased summer low flows, and infrequent bankfull and out-of-bank flows. The Willamette Flow Project partners anticipate the study will be expanded in the future to encompass the other major tributaries controlled by USACE dams.
  • Prepared for Sustainable Rivers Project of the Nature Conservancy and the U.S. Army Corps of Engineers.
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  • TABLE OF CONTENTSIntroductionWillamette River Flow Project.............................................................................................1Goals and Objectives .........................................................................................................2Summary Report Purpose ..................................................................................................3BackgroundStudy Area Description.......................................................................................................5Hydrologic Network and Discharge Regime .......................................................................6Physical Processes and Conditions: Flow RequirementsWater Quality .....................................................................................................................9Temperature..................................................................................................................9Nutrients and Toxic Pollutants .......................................................................................11Sediment and Turbidity..................................................................................................12Geomorphology and Floodplain Inundation ........................................................................14Biological/Ecological Conditions: Flow Requirements .....................................................20Terrestrial Vegetation .........................................................................................................20Cottonwood and Willow.................................................................................................22Ash, maple and alder.....................................................................................................23Invasives .......................................................................................................................24Terrestrial Vertebrates: Birds and Mammals.......................................................................28Aquatic Invertebrates .........................................................................................................31Short-lived species ........................................................................................................32Long-lived species.........................................................................................................32Aquatic Vertebrates............................................................................................................34Amphibians and Reptiles...............................................................................................34Western pond turtles ..............................................................................................34Red-legged frog......................................................................................................35Bullfrog ...................................................................................................................35Fish ...............................................................................................................................38Spring Chinook.......................................................................................................39Lamprey .................................................................................................................40Coastal cutthroat trout ............................................................................................41Oregon chub...........................................................................................................41Large-scale sucker .................................................................................................42Large- and smallmouth bass...................................................................................43Summary...............................................................................................................................47Bibliography ..........................................................................................................................48Tables....................................................................................................................................58Figures..................................................................................................................................100Appendices...........................................................................................................................199
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