Instream boulder placement will provide structure and habitat in the main stream channel and terrace log placement will activate the side channels in high flows. Placement of log and rock material will sort and accumulate spawning gravels which will provide spawning habitat for spring Chinook salmon.
Placement of boulders clusters will provide
places for fish to take refuge in the flashy flows
of Mosby Creek. The boulders will cause localized scour, which will improve instream habitat diversity while sorting gravels. Over time, the stream may cool in temperature due to this gravel accumulation and improved hyporheic exchange of cooler water. This project is anticipated to affect approximately 2 stream miles of aquatic habitat.
Mosby Creek is a headwater tributary to the Coast Fork Willamette River. Flowing northwest from the low-elevation Cascades, Mosby Creek is a free-flowing stream that drains a watershed of approximately 95 square miles of predominantly Douglas fir forest uplands. The majority of the Mosby Creek watershed (53%) is owned and managed by Weyerhaeuser Company, which purchased the land from Georgia Pacific in the 1970s. The Bureau of Land Management (BLM) owns approximately 35% of the Mosby watershed. The remaining 12% is in private ownership managed as rural residential and small woodlots. The southernmost Mosby ridgeline is the boundary between the Willamette and Umpqua Watersheds.
Fish species found in Mosby Creek include spring Chinook salmon, large-scale suckers, cutthroat trout, rainbow trout, sculpin, dace, whitefish, redside shiners, and brook and Pacific lamprey. Spring Chinook salmon are native to the Coast Fork Willamette Watershed but the Mosby Creek population was not as abundant historically as in neighboring tributaries (NOAA, 2008). A 2005 Oregon Department of Fish and Wildlife (ODFW) screw trap caught spring Chinook. In addition, ODFW released spring Chinook into Mosby Creek in 2006 and 2007 and placed Chinook carcasses in Mosby Creek throughout the past five years to increase nutrient levels in the stream.
Channel Conditions: A Design Challenge
Mosby Creek is unique because it is a waterway that is not dammed; it has the geomorphic potential to be restored to an extent that it could support more resident trout and other native fish species (including Pacific lamprey) and likely a recovered spring Chinook salmon run. There are specific geomorphic factors limiting aquatic habitat recovery. These factors include bedrock-dominated reaches and high width-to-depth ratios. Our project, OWEB #210-3001, the Mosby Creek Spring Chinook Re-establishment Project, sought to make specific channel improvements to remedy these geomorphic limiting factors.
The Mosby Creek watershed was mostly logged prior to the implementation to the Oregon Forest Practices Act that required buffers on streams that support fish. The floodplain and some of the mainstem of Mosby Creek were mined for gravel for road building and maintenance. Many of the side tributaries were realigned and straightened to facilitate road building and logging in the 1950’s and 1960’s.
A comparison of riparian vegetation and channel morphology from 1955 to 1993 found isolated but severe channel widening in the upper watershed following the 1964 flood (Stark, 1995). “Nearly three quarters of the Mosby Creek basin riparian stands were in good condition to provide large woody debris (LWD) to the stream channel in 1955. This fraction declined to one-sixth by 1979 and has remained at this proportion to the present (Stark, 1995). By 1993, riparian vegetation on tributaries improved canopy closure on tributaries, but large wood recruitment potential remained moderately poor on both the mainstem and tributaries. As of 1993, riparian canopy closure on the mainstem segments had only moderately recovered (Stark, 1995).
Mosby Creek is a hydrologically “flashy” system with flow levels that change very quickly. In the upper basin, high flows of between 5000-7000 cubic feet per second (cfs) can occur during rain-on-snow events. Conversely, summer base flows of less than 10 cfs were recorded within the project sites. Both high and low flows in Mosby Creek have consequences for fish habitat. The high peak flows push fallen large wood out of the system. The low flows limit fish passage in the summer and exposed bedrock contributes to elevated water temperatures.
The chosen design considers flashy flows, the lack of structural components in the stream, and exposed bedrock and addresses these challenges with a solution suitable to the problem. The rock weirs result in habitat quality improvement by creating spawning and rearing habitat for spring Chinook and summer Steelhead by increasing connectivity to the floodplain and creating pools. Due to the risk of temperature being a primary limiting factor for trout and salmon, the project team is pleased to identify a design solution that could both enhance spawning and rearing habitats as well as potentially reduce peak water temperatures.
Spring Chinook Habitat Project Phases I & II
Phase I: Summer 2011
Six weirs and associated rock and wood structures were placed in August 2011 in five locations in Upper Mosby Creek on BLM and Weyerhaeuser property. These structures used boulders as the primary structural element to construct full spanning weirs and various other boulder arrangements, including clusters and barbs. These features are capable of capturing gravel, creating rearing habitat, and are unlikely to be swept out at high flows. Boulder structures may also mitigate thermal extremes through reconnecting the hyporheic zone in bedrock-dominated areas adjacent to the project area. Our Phase I results are showing a temperature change of ~2 degrees F across conditions local to each weir. We estimate our zone of influence from our project to be two stream miles.
Phase II: Summer 2014
Our proposed Phase II project includes five rock weirs at two locations. By incorporating lessons about how these structures function in Mosby Creek, we are moving into a design in which each rock weir is inter-related to the one upstream. This allows a contiguous block of improved habitat, including gravels, pools, and floodplain connectivity to extend across the entire project area.
- 463 Stream miles in Mosby Creek Watershed of which 95 miles (20%) are potential fish bearing habitat (BLM).
- Total area of Mosby Sub-Basin: ~ 95 sq. miles / ~ 60,982 acres
- Land Ownership in Mosby Creek:
- Weyerhaeuser: 51 sq. miles / 33,188 acres (53%)
- BLM: 33.6 sq. miles / 21,500 acres (35%)
- Other Private: 10.4 sq. miles / 6,342 acres (12%)
Links and Document Downloads
- mosby poster.jpg3.28 MB07/03/2015, 13:40
- Mosby Creek Spring Chinook Re-Establishment Project Final Report,” by Coast Fork Willamette Watershed Council, Feb. 2012
- “History of Fish Use in Mosby Creek” factsheet, by Eric Moberly
- “Free Flowing Rivers Potential to Augment Salmon and Steelhead Recovery Efforts,” by R.J. Danehy, Weyerhaeuser Forestry Research, Springfield, OR, 2010
- “Mosby Creek Watershed Analysis,” U.S. Department of Interior, Bureau of Land Management, November 2000.