Lower South Fork McKenzie River Floodplain Enhancement Project

Recognized as the 2018 regional winner of "Innovative Restoration," the Lower South Fork McKenzie River Floodplain Enhancement Project is the largest aquatic restoration project on the forest to date, connecting 4.5 miles of floodplain at the confluence of Cougar Creek and the South Fork of the McKenzie River. Opening up the floodplain will restore, to the extent practicable, the physical, chemical, and biological processes that maintain a healthy and resilient floodplain ecosystem. A well-functioning floodplain will provide diverse habitat for aquatic and terrestrial communities, including threatened spring Chinook salmon and bull trout, Pacific lamprey and other native fishes, western pond turtle, amphibians, beaver, and waterfowl.

This project is an important part of regional efforts to restore habitat for spring Chinook salmon and bull trout in the Willamette River Basin. The project shifts a levied and channelized river that keeps all its flow confined to a single channel to one that easily spreads water across several hundred acres of floodplain, increasing surface and subsurface flow, creating fish habitat, and improving groundwater recharge.

Floodplain treatment graphic

The project is located in the South Fork McKenzie River Watershed, approximately 50 miles east of Eugene, Oregon. The project area is approximately 784 acres in size and includes the main stem channel and all current and historic side channels and floodplain. Most of the project area is under Forest Service ownership, except for approximately 32 acres owned by the US Army Corps of Engineers.

Historically, the lower South Fork was a large alluvial valley with a complex network of side channels and frequently inundated floodplain. It was a depositional zone for much of the sediment, wood, and nutrients coming out of the South Fork drainage. As is typical of low gradient, alluvial valleys, this area was very biologically productive. According to 1937 surveys, the South Fork was the most important tributary for spring Chinook salmon spawning and the confluence area was known as a “bull trout paradise” that was intensively fished.

Unfortunately, the South Fork has been significantly altered in the last century. Cougar Dam was built by 1963 for flood control and power generation. Associated with construction of the dam was straightening and channelization of the lower river with substantial amounts of levee, fill, and riprap material. In addition, large wood was removed from the river for timber and navigation purposes. These combined activities have limited the sediment and wood supply, altered the flow regime, reduced channel complexity, and restricted off-channel and floodplain connectivity.

a comparison of current and future habitat
Ideal fish habitat includes streams with side channels, logs, and overhanging branches, which produce meanders, pools, riffles, shelter, and shade. Native fish, such as salmon, also require small gravel (1/2 to 3 inches in diameter) for spawning. The top photo shows a steam channel in the project area where fish are unable to spawn due to the size of materials. The bottom photo shows an area where water is being diverted and wood such as logs and root wads have been placed. It gives an example of what most of the 125-acre project area will look like after restoration. Areas like this will provide high quality habitat for native fish and beavers.

Habitat surveys reveal very low pool habitat (12%) and wood abundance. The dominant substrate in both pools and riffles is cobble – too large for spawning. Fine sediment is no longer deposited onto floodplains, limiting nesting habitat for turtles. Although Cougar Dam presents a major obstacle to floodplain restoration by altering flows and blocking wood, sediment, and nutrients, management actions can significantly improve conditions.

An employee measuring and counting wood

By removing levees, fill, and riprap and adding large wood and sediment we will:

  • Increase pools and pockets of slow water so fish can feed and rest,
  • Provide cover from predators,
  • Retain gravels necessary for fish spawning and fine sediment needed for lamprey rearing and western pond turtle nesting,
  • Maintain a well-connected floodplain with abundant side channels, ponds,and wetlands for amphibians, beaver, waterfowl, and fry and juvenile fish rearing.

Employees in hard hats plan out work dayRecent studies indicate that stream biodiversity and productivity is greatest in streams that have a complex network of side channels (Cluer and Thorne 2013, Martens and Connolly 2014). Complex streams with a well-connected floodplain can be up to 250% more biologically productive than single thread channels (Bellmore et al. 2013). A review of projects designed to restore floodplain connectivity and side channel habitat increased salmon and trout production by 27-34% (Ogsten et al. 2014). By adding large wood we will be creating habitat features (pools, cover, gravel, side channels) that are important for native fish and wildlife.

Learn more at http://www.mckenziewc.org/?page_id=884.

Oregon Watershed Enhancement Board
Eugene Water & Electric Board
McKenzie Watershed Council