December 9, 2020: Humboldt County will be re-noticing the public comment period in January or February, 2021. All comments already received by the County will remain part of the public record.
October 7, 2020: Project Status Update Presentation
SRF Executive Director Dana Stolzman and Stillwater Sciences engineer Joel Monschke provide
an update on the Marshall Ranch Flow Enhancement project (video of slides).
For more resources and updates, please click here.
Since 2013, Salmonid Restoration Federation has been conducting low-flow monitoring in Redwood Creek, a
critical tributary to the South Fork Eel River. The South Fork Eel River is considered one of the highest priority
watersheds in the state for flow enhancement projects. Forested tributaries like Redwood Creek provide refugia
habitat for threatened juvenile coho salmon but suffer from the cumulative impacts of legacy logging and
unregulated water diversions.
With funding from the Wildlife Conservation Board and the California Department of Fish and Wildlife, SRF and
Stillwater Sciences have been exploring the feasibility of various streamflow enhancement opportunities in Redwood
Creek. Stillwater Sciences conducted a feasibility study in a segment of the watershed that helped to identify priority
projects that could improve summer flows.
Stillwater Sciences prepared conceptual designs for off-channel rainwater catchment ponds that could improve
water security for individual parcels but would require wide and coordinated participation in order to
measurably improve flows. After much research and reconnaissance, the SRF and Stillwater project team
determined that the greatest opportunity to improve streamflows was to work cooperatively with the Marshall
Ranch, the largest private parcel in the watershed that has been stewarded by the Marshall family since the
1800s and is now fully protected under conservation easement.
Marshall Ranch Flow Enhancement Project
The Marshall Ranch where the proposed project is situated is a rare example of a large, contiguous land area in the
Redwood Creek watershed. The 2,942-acre ranch comprises 34 legal parcels all under one family ownership,
which has been managed sustainably for timber production and livestock since the 1880s while also providing
extensive habitat for fish and wildlife as evidenced by the presence of coho, Chinook and steelhead.
The proposed Marshall Ranch Flow Enhancement Project is being designed to significantly improve
Redwood Creek dry-season conditions. A 15.3 million gallon off-channel pond is proposed to store winter
runoff and release approximately 50 gpm of cool clean water into 5.5 miles of Redwood Creek during the
five-month dry season. This water input is expected to have a significant and measurable benefit to salmonids
and other aquatic habitat in Redwood Creek. A fire suppression component is also being designed into the
project. The pond will be accessible for helicopters to dip their buckets and a fire hydrant gravity fed from the
pond will be available for access by fire engines during emergencies. This project includes a 7.5 KW solar array grid intertie
system and associated infrastructure to offset annual power use and ensure that the project itself including
valves, sensors, internet connection, etc. will be operational in a power outage.
Selection of the off-channel pond site has been guided by office- and field-based assessments of a significant
portion of the Redwood Creek watershed. Based on these assessments, the proposed pond location is uniquely
suited for the project due to the following factors: 1) the project area is comprised of a broad area with gentle
topography, 2) the site is not within the Redwood Creek floodplain or within the potential Redwood Creek
channel migration corridor, 3) there are no watercourses, wetlands, trees or other sensitive plant species within
the proposed pond footprint so environmental impacts are minimal, 4) the pond site is located at an elevation
with enough pressure head to deliver the entire pond volume to Redwood Creek by gravity, and 5) the Marshall
Ranch LLC (landowner) is fully supportive of the project.
Due to the size of the project and nearby downslope neighbors, a detailed analysis of site conditions, pond
design features, and potential failure mechanisms is required. To support the project design process, further in-
depth analyses of the site and its surroundings were conducted to ensure that the pond would be stable for the
long-term. The site is a unique geomorphic feature within the Redwood Creek watershed since it is a
Pleistocene fluvial terrace (between 10,000 to 2 million years old). The geotechnical evaluation for the project
confirmed this finding with boreholes consisting of hard shale bedrock overlaid by sandy gravel deposits (old
sediment from when Redwood Creek was flowing on the terrace approximately 80 ft higher in elevation than
the current creek level). On top of the pre-historic creek deposits, 10 to 20 feet of alluvial fan material has been
slowly deposited over the last 10,000 years from the upgradient hillslope and small swale. These multiple lines
of scientific evidence supporting terrace stability provided the basis for the pond design prepared in September
2019 and accompanied initial CEQA application documents.
2020 Design Modifications
During the CEQA public comment period, concern was raised by downslope landowners that the proposed pond
and associated grading and infrastructure may not meet the desired level of long-term safety, especially during
the rare case of a large rainfall event coupled with a large magnitude earthquake. Based on these concerns,
additional analyses have been conducted including further assessment of potential pond failure mechanisms and
seismic slope stability analyses under worst-case current and proposed conditions. Based on these analyses,
several significant design revisions are being considered that will greatly reduce risk associated with pond berm
and spillway failure. The design modifications include:
- Lowering the pond elevation by eight feet will significantly reduce the weight on the existing terrace
below current conditions. Now, the vast majority of pond volume will be created by excavating a large
trough in the terrace.
- Relocation of the pond spillways and changing the initial culvert spillway (in the previous design
phase) to a rock-lined spillway will increase longevity and reduce long-term maintenance costs.
- Installation of a pond liner, French drain, and subsurface restrictive barrier to reduce seepage
Incorporating the proposed design modifications is expected to lower the risk of slope instability that could
impact the downslope landowners compared to current conditions, as indicated by slope stability analyses. This
is a result of the proposed project significantly lowering the water table within the upper terrace and stabilizing
the central gully. According to the lead Stillwater Sciences engineer, Joel Monschke, “The pond is set back a
significant distance from the slope break and is primarily constructed through excavation into the terrace. There
is no plausible mechanism for massive pond failure.” This finding is supported by geologists from Stillwater
Sciences and an engineering geologist and geotechnical engineer from SHN who have analyzed the proposed
Proposed design modifications have been reviewed by technical advisors from the California Department of
Fish and Wildlife, North Coast Regional Water Quality Control Board, NOAA Fisheries, and the State Water
Board. Revised project documents were submitted to Humboldt County Planning Department in September, and the
County will soon officially notify downstream neighbors of a 30-day public comment period. After
that 30-day comment period, there will be a public hearing scheduled for any interested parties to attend via
Zoom. SRF will do our due diligence to notify downstream landowners and the Redwood Creek community
about the public comment timeline and the significance of this project.