Willamette River Floodplain Restoration Study Draft Integrated Feasibility Report/ Environmental Assessment
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WILLAMETTE RIVER FLOODPLAIN RESTORATION STUDY DRAFT INTEGRATED FEASIBILITY REPORT/ ENVIRONMENTAL ASSESSMENT Lower Coast and Middle Fork Willamette River Subbasins March 2013 Prepared by: U.S. Army Corps of Engineers Tetra Tech, Inc. Portland District Portland, OR Willamette River Floodplain Restoration Study Draft Report This page was intentionally left blank to facilitate double-sided copying. Willamette River Floodplain Restoration Study Draft Report EXECUTIVE SUMMARY This feasibility report presents the results of a U.S. Army Corps of Engineers (Corps) feasibility study undertaken to identify and evaluate alternatives for restoring natural floodplain functions in the lower Coast and Middle Forks of the Willamette River. This report provides documentation of the plan formulation process to select a recommended restoration plan, along with environmental, engineering, and cost details of the recommended restoration plan, which will allow final design and construction to proceed following approval of this report. The study area is the Willamette River Basin of western Oregon. The Willamette River is a major tributary of the Columbia River and is the tenth largest river in the United States, based on average annual flow. This study is being conducted in phases due to the large size and complexity of the Willamette River Basin. Phase 1 of the study involved the development of a framework level plan for the entire Willamette Basin – the Willamette Subbasin Plan (WRI 2004); which documented conditions in the basin and strategies to recover fish and wildlife species as part of the Columbia River Basin Fish and Wildlife Program. Phase 2, the subject of this report, is a feasibility study of floodplain restoration opportunities in the lower Coast and Middle Forks of the Willamette River. The Coast and Middle Forks study is an initial study to develop the process for identifying and evaluating restoration alternatives to function in the dynamic floodplain environment. The process developed in this phase will then be used in the analysis of other subbasins within the larger Willamette River Basin. The Coast Fork and Middle Fork of the Willamette River merge upstream of Eugene, Oregon to form the mainstem Willamette River (see Figure ES-1). The lower Coast and Middle Forks and their floodplains downstream of Cottage Grove and Dexter Dams, respectively are of interest for floodplain restoration because many areas of the floodplain are relatively undeveloped and the construction and operation of Corps’ revetments and the upstream dams have had adverse effects on floodplain functions which makes Corps involvement appropriate. The lower Coast and Middle Forks of the Willamette have substantial opportunities for floodplain restoration. This study has involved a large number of stakeholders including agencies, watershed councils, academic researchers and other organizations. From the outset, this stakeholder group has helped scope and refine the study. The restoration goal and objectives developed with the stakeholders for this study are outlined below. The goal and objectives have guided the plan formulation process and selection of a recommended restoration plan. Coast and Middle Fork Floodplain Restoration Goal and Objectives Restore natural floodplain ecosystem function and Restoration Goal condition to the Coast and Middle Fork Subbasins. Restoration Objective 1 Increase channel complexity and diversity Restoration Objective 2 Restore connectivity of river to floodplain habitats Restore native floodplain habitats, including Restoration Objective 3 cottonwood gallery forests, riparian and wet prairie habitats ES-1 Willamette River Floodplain Restoration Study Draft Report Figure ES-1. Coast Fork and Middle Fork Subbasins ES-2 Willamette River Floodplain Restoration Study Draft Report The baseline condition in the study area is degraded for native fish and wildlife and has much reduced floodplain functions. Causes for this degradation include the construction of upstream dams that have modified flows and raised water temperatures, construction of revetments that reduce channel migration and the formation of habitats, development of the floodplain, removal of riparian vegetation, removal of wood from the river channels, blocking access to off-channel habitats, and the widespread presence of non-native species. Forty-three potential floodplain restoration sites were identified in consultation with the stakeholder group and were initially developed at a conceptual level. Two or more scales of restoration were considered at each site: 1) a minimum level of restoration that included measures such as removal of invasive species, replanting with native riparian and floodplain vegetation, and placement of large wood in the floodplain; and 2) a maximum level of restoration that included the measures from the minimum scale plus other features such as excavation of channels, removal of revetment, floodplain grading, restoration of gravel mined ponds, installation of engineered log jams, culverts or bridges, and other features. The majority of the potential sites were visited to collect data on existing conditions and to confirm if restoration measures were appropriate, and/or to add additional measures. Preliminary cost estimates were developed for each site and scale of restoration, along with calculation of habitat benefits that could accrue from restoration measures. Habitat benefits were calculated using a multi-species Habitat Evaluation Procedures (HEP) model. The preliminary costs and habitat benefits were used in a Cost Effectiveness and Incremental Cost Analysis (CE/ICA) to identify alternatives that provide high levels of habitat benefit relative to the costs. The CE/ICA was a primary element used to select the recommended restoration plan. A recommended restoration plan was selected that includes floodplain restoration on five sites (Figure ES-2). All five sites selected for restoration have been subject to gravel mining in the past and include multiple gravel mined ponds. Restoration would include restoration of the gravel mined ponds by grading on-site sediments and topsoil back into the ponds to create wetland and shallow water habitats, excavation of connector channels between ponds and to connect to the rivers via backwater channels, removal of invasive species, plantings with native wetland, riparian and floodplain vegetation, placement of large wood in the ponds and floodplain, installation of engineered log jams in the river adjacent to the sites, removal of debris and revetment materials, and installation of culverts at road-crossings to remain on site. The effects of the recommended restoration plan would be beneficial in the long-term by restoring native fish and wildlife habitats, increasing floodplain connections and storage on a frequent basis, promoting the natural formation of gravel bars and side channels by the installation of engineered log jams and removal of revetment material and providing fish access into off-channel and floodplain habitats. There would be no rise in the 100-year water surface elevation. During construction there could be temporary adverse effects such as the potential for increased turbidity, release of pollutants, and handling of fish. These effects would be minimized by providing erosion and pollution control best management practices (BMPs) and conducting all fish salvage and removal activities according to state and Federal requirements. A number of conservation measures would be implemented during construction to minimize effects to listed species. ES-3 Willamette River Floodplain Restoration Study Draft Report Figure ES-2. Location of Project Sites Included in Recommended Restoration Plan. Overall, the recommended restoration plan would provide frequent hydrologic connections to nearly 600 acres of floodplain habitats; restore off-channel habitats for fish rearing and refuge; significantly improve the quality of a variety of natural floodplain, wetland, and riparian habitats, reduce invasive species, and reduce fish stranding and mortality that can currently occur following flood events. This plan would also provide significant benefits to listed fish species that occur in the Upper Willamette basin. The recommended restoration plan is proposed for implementation at a total project cost of $48,882,222 to be cost-shared 65% Federal and 35% non-Federal. The non-Federal sponsor is the Nature Conservancy. ES-4 Willamette River Floodplain Restoration Study Draft Report TABLE OF CONTENTS 1. STUDY BACKGROUND ........................................................................................... 1 1.1. Study Overview .................................................................................................... 1 1.2. Study Authority .................................................................................................... 1 1.3. Planning Process ................................................................................................... 1 1.4. Reconnaissance Study ........................................................................................... 5 1.5. Feasibility Study Scope and Objectives .................................................................... 6 1.6. Project Purpose and Need....................................................................................... 9 1.7. Study Area ........................................................................................................... 9 1.7.1. Coast Fork Willamette River Subbasin ................................................................