South Fork Floodplain Enhancement Phase I


This project is located within the Cougar Creek-South Fork McKenzie River Sub-watershed. The project area encompasses the lower 4.2 miles of the South Fork McKenzie River from the base of Cougar Dam to the confluence with the McKenzie River and is approximately 780 acres in size. It includes two large alluvial valleys, separated by a transport reach, with another transport reach right below cougar. Most of the project area is under USFS ownership, except for approximately 40 acres owned by the USACE. This comprehensive stream design is for approximately 4.2 miles of the lower South Fork McKenzie River. 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 is designed to rehabilitate to the maximum extent practicable the physical, chemical, and biological processes that are impaired by the Cougar Dam

History

•Cougar Dam cut off wood, sediment, and nutrient supply, altering flow and temperature regimes

•Levees, riprap, and fill, straightened and channelized river

•Stream cleaning and logging removed wood and left legacy roads and berms

•Channel incision (up to 14 feet in places

•< 30% of historic floodplain being utilized

•Lack of spawning, rearing, and foraging habitat for spring Chinook salmon, bull trout, Pacific lamprey, and other native species

 

Design

• The aim is to reconstruct a valley bottom surface that is connected at base flow by removing artificial features, filling incised channels, and adding abundant large wood throughout the valley bottom

•Stream power is distributed laterally across a large portion of the valley, rather than concentrated in a high capacity channel, which promotes the deposition and storage of mobilized sediments, organic material, nutrients, and organisms

•Habitat is then naturally formed with the passage of floods, sediments, and organic material over time; no channels or static habitat features are constructed

•Stage 0 systems are highly dynamic, complex, and resilient multi-thread systems with shallow alluvial aquifers and an abundance of roughness elements

Implementation

•Worked with USACE to divert entire South Fork (330 cfs) into relic side channel

•Removed roughly 85,000 cubic yards of sediment material from 16 acres of floodplain

•Aggraded 0.7 linear stream miles 1-5 feet with redistributed material

•Significant fish salvage effort with 
ODFW and volunteers

•Placed ~ 3,000 pieces of large wood throughout disturbed areas and relic floodplain channels

•Raised water table and increased base flow wetted area by 350%