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Lower South Fork Floodplain Enhancement Project

Project Number

South Fork McKenzie River aerial view
South Fork McKenzie River fish salvage
South Fork McKenzie River sediment placement
South Fork McKenzie River wood jam

218-3004

Grantee

McKenzie Watershed Alliance (MWA)

Partners

U.S. Forest Service (USFS), Oregon Department of Fish & Wildlife, U.S. Army Corps of Engineers

Project Overview

The installation of Cougar Dam in 1963, placement of levees and berms, harvest of riparian forests, and active removal of in-stream wood combined to transform the lower South Fork McKenzie River (South Fork) from a diverse and dynamic ecosystem to a simplified, largely single thread channel with limited habitat for spring Chinook salmon, bull trout, and other native species. Cougar Dam blocked access to approximately 20 miles of the mainstem South Fork for anadromous and migratory fish. Cougar Dam significantly altered downstream temperature conditions on the South Fork. This change had negative impacts on both fish migration patterns and salmonid fry emergence.

The Lower South Fork Floodplain Enhancement Project is a multi-phase effort to improve river function on 4.5 miles and over 600 acres of floodplain on the South Fork below Cougar Dam. Phase I reconnected over 200 acres of floodplain on US Forest Service (USFS) land downstream of Cougar Dam. The South Fork was diverted to a relic floodplain channel and was dropped to 330 cfs during the project.

Nearly 0.7 miles of eroded areas of the SFMR were filled in and built up with over 85,000 cubic yards of sediment removed from berms and floodplain deposits. Nearly 3,000 pieces of large wood were placed throughout the floodplain by ground-based equipment and helicopter. BCI Contracting Inc (BCI) used excavators to tip all trees with rootwad intact and clear all undergrowth and slash (woody debris). Approximately 300 pieces of wood were staged for helicopter placement to avoid cutting skid trails into undisturbed portions of the floodplain. The remaining trees and slash were placed via ground transport. 

USFS and MWA personnel spent 3 days using backpack electroshocking equipment to salvage fish within side channels, backwaters, and mainstem channel margin areas. All fish captured were identified to species and/or family, numbered, and released outside of the project area. The SFMR is once again flowing over its floodplain and forming a range of diverse pool, backwater, and wetland habitats.

Photos

Top: Aerial image of end of the project area and upstream untreated reach from diversion channel looking east.

2nd: Block net and seine fish salvage operations in isolated pool.

3rd: Bulldozer placing and grading sediment within fill area in the old main stem South Fork channel.

Bottom: Large wood jam.

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