Estuary News

March 2021

Fixing a Dysfunctional Marsh on Sonoma Creek

Restoration projects, like species, evolve. The Sonoma Creek Enhancement Project, originally about mosquito control, has shown itself to be a boon to special-status tidal marsh wildlife as well. More than a decade of adaptive management actions made that happen. 

The existing marsh, formed rapidly beginning in the 1960s by deposited sediment, lacked the dendritic channels of a mature marsh. High tides brought in water that pooled in a central basin and didn’t drain out, providing breeding habitat for mosquitos. The disadvantages of chemical treatment prompted land managers to look for alternatives.  So in the 2000s, the Marin-Sonoma Mosquito and Vector Control District teamed up with the US Fish & Wildlife Service (the land manager), Audubon California, and environmental scientists Daniel Gillenwater and Stuart Siegel to improve tidal circulation in the dysfunctional marsh.

Transition ramp 2015
Transition ramp 2019

Their proposed solutions included creating a mile-long mid-marsh channel to improve tidal action and smaller channels to stagnant areas, and, for sea level rise resiliency, using dredge material from the channels to build a transition ramp on the land side and mounds on the marsh plain. Placing fill on existing wetland for the ramp complicated the permitting process, but encouraged the San Francisco Bay Conservation and Development Commission to revisit fill regulations and set precedent for future projects. “The Sonoma Creek and Aramburu Island projects broke the mold on that,” recalls Gillenwater.  

Partial construction in 2015 (Phase 1) improved tidal flushing enough that mosquito treatments could be scaled down. But ongoing monitoring at the site revealed issues that required tweaking: “We knew the channel wasn’t addressing all the drainage problems.” In 2018 the District dug 2000 feet of adaptive-management side channels.  That left other unfinished business for a Phase 2 effort: extending the central channel to its full design length and removing Phase 1 construction debris that clogged it. Funding came from mitigation money from PG&E and Tesoro earmarked for use in the local area designated in the recovery plan for the endangered Ridgway’s rail. “We had a shovel-ready project,” says Meg Marriott, acting manager of the San Pablo Bay National Wildlife Refuge.

Remodeled and extended channel. Photo: Dan Gillenwater

With general permits already in place, final Phase 2 design work started last spring, just as the COVID-19 pandemic struck. “We went from concept to construction in six months,” engineer Melissa Carter of the consulting firm Environmental Science Associates explains. Despite the challenges of the pandemic, which allowed only the use of essential workers, and the wildfires of autumn, construction work got done in nine days.

As part of Phase 2, the team had to decide what to do with the new material dredged from the channel extension: extend the ramp or add marsh mounds? Constructability gave the mounds an edge. The Phase 1 mounds, planted with pickleweed from the marsh and other species like gumplant and sea-lavender , provide high-tide refugia for the Ridgway’s rail, California black rail, and salt marsh harvest mouse.  The gumplant and other plants were grown and planted by Students and Teachers Restoring a Watershed (STRAW) volunteers at the Refuge’s nursery

STRAW team ready to work. Photo: STRAW

Wildlife response has been gratifying. Marriott says Ridgway’s rails lived near the project area before Phase 1 and may now be colonizing the marsh; a pair was heard calling last year. Harvest mouse populations are stable in a nearby reference area but increasing within the project area. The planted mounds offer vertical structure the mice need to shelter from predators during high tides. The transition ramp, also planted by STRAW volunteers and colonized by native plants, functions as existing habitat as well as a hedge against a future of rising seas and extreme weather events.

With improved drainage and enhanced landscape, the marsh has been transformed. “Before Phase 1 it was a barren, shallowly-ponded moonscape with scraggly vegetation and clouds of mosquitos,” says Carter. “Now there’s pickleweed as far as the eye can see: knee-high, thigh-high, flourishing.”  And mosquitos pose less of a threat to humans and wildlife.

Once again, adaptive management helps work out the bugs.

Prior Estuary News Stories

Highway 37: the Road to Restoration, Sonoma Creek Spotlight, June 2020

Other Links

Sonoma Creek Enhancement Project

Top Photo: Melissa Carter, ESA

About the author

Joe Eaton writes about endangered and invasive species, climate and ecosystem science, environmental history, and water issues for ESTUARY. He is also "a semi-obsessive birder" whose pursuit of rarities has taken him to many of California's shores, wetlands, and sewage plants.

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