More than two thirds of the Columbia Slough floodplain is located behind levees. The first levees were constructed between 1919 and 1921 by local residents, in conjunction with the Multnomah County Drainage District and the U.S. Army Corps of Engineers. Additional construction has since heightened and strengthened levees to prevent flooding when water on the Columbia River, Columbia Slough and Willamette River would otherwise flood this low lying area. The levees initially protected farmland, sawmills, and Portland's stockyards and slaughterhouses. Today the levees protect more than 10,000 acres of land, 2,000 landowners, and Portland's airport.
The purpose of this project is to restore aquatic and riparian habitat along nearly seven miles of the middle and upper slough segments. The project does not affect flood control.
Section 1135, P.L. 99-662, authorizes the Secretary of the Army to modify the structure and operation of water resource projects to improve the quality of the environment in the public interest in areas where Corps projects have affected environmental conditions.
The 1135 Restoration Project will improve water quality and create and restore wetlands along a seven-mile segment of the Columbia Slough. The project consists of three main components:
- Creation of wetland benches and a meandering channel between NE 18th Avenue and NE 158th Avenue. Dredge material will be placed along opposite sides of the channel to create wetland benches and a meandering low water channel. The benches will be planted to provide emergent wetland and riparian scrub-shrub wetland vegetation. Little emergent marsh habitat is currently available along the main slough, primarily due to the steep banks and narrow channel along most of the project area.
- Replacing five culverts in the Columbia Slough to improve water flow, temperature and quality. Undersized, blocked, deteriorated, and high invert elevation culverts restrict flow. These flow restrictions increase hydraulic retention time and raise water surface elevations. The stagnant water provides favorable conditions for high water temperatures and severe algal blooms, leading to poor water quality and aesthetics. Culverts will be replaced to decrease water residence time in the slough to improve water quality conditions.
- Enhancing a nine acre wetland marsh and restoring nine acres of adjacent riparian woodland habitat by removing invasive species and planting native species. The marsh was drained for agriculture and is now a reed canary grass wetland with little habitat variety. Restoration will focus on recreating wetland and open-water habitat. The riparian woodland habitat restoration will focus on increasing forest cover, improving age distribution of trees, and snag recruitment. These projects are located in the upper slough watershed.