Sedimentation and Dredging

Marshlands are transition zones between the land and ocean and evolve over geologic time with tectonic plate movement, erosion, and sea level changes. Sediments (small soil and organic particles) flow up the Petaluma River and wetlands from ocean tides and flow down from soil erosion and landslides in the watershed. In addition, the Sacramento and San Joaquin Rivers transport large volumes of sediment through the Sacramento-San Joaquin Delta to San Pablo Bay, particularly in the rainy season, which can then enter the mouth of the Petaluma River. These three sources of sediments significantly increase the total load delivered to the water channel network affecting both navigation and water-carrying capacity. Tidal inflow and outflow, twice every 25 hours, constantly moves sediments up and down the river, making the river water “muddy”, reducing sunlight penetration. Total accumulation also depends on sea level changes so expanding oceans from increased temperature and ice melting will bring in more sediment, raising the entire marsh over the next 50 years by at least one foot.

Vegetation traps sediments and builds soil. Cordgrass is a major species that does this at the water edges. In the higher marsh, both Pickleweed (Salicornia virginica) and Saltgrass (Distichlis spicata), the major higher marsh dominants, are stress tolerant species that can maintain their productivity under adverse conditions irrespective of local sedimentation deposition. The value of vegetation adapted to variable and elevated soil salinity is to substantially contribute to organic matter accumulation in marsh soil profiles.

Dredge spoils output

Dredge spoils output

Organic matter is dissolved/suspended in water and attaches to soil and sediments. Decaying plants and animals also contribute larger organic particulate
matter building the base of the detritus food web. Detritus is all the non-living organic matter and the associated microbial community. The detritus food web turns over more carbon than rainforests making saltwater marshes highly biologically active and productive ecosystems. The complex relationships between the biotic and abiotic systems including water and sediment flows, vegetation, and the detritus food web have evolved over millennia, so the salt marsh ecosystem is robust, adapting to changes, and capable of surviving eventful impacts. These relationships also involve feedback systems that influence sediment deposition, soil formation, and marshland elevation so that even with the large volume of movement, the overall changes over a long time are slow.

Digging deeper and wider channels and straightening the curves to improve riverboat traffic changes the natural system. Nature will fill in any sediment we remove from the bottom of the channel to bring it back to its historical natural balance. Dredging is needed to protect against flooding of our downtown area while permitting more robust river traffic. Without dredging the river would rapidly fill with silt, and it would no longer be navigable for commercial or recreational vessels. Continuous dredging, year after year, is costly, so we need to balance the economic value and review its sustainability.

Dredging tube on the Petaluma River

Dredging tube on the Petaluma River

Problems associated with siltation and flooding, recognized over a century ago, still exist today. Since the 1880s the Army Corp of Engineers (ACOE) improved and maintained the Petaluma Creek for navigation. The Corps’ dredging project from 1930 to 1933 provided for a 200-foot wide, 8-foot deep channel for 33,000 feet across the mudflats in San Pablo Bay to the mouth of the Petaluma Creek. For the next 69,000 feet upstream to Western Avenue in the City of Petaluma, the channel was widened 100 feet and deepened 8 feet. Included in this part of the project was 300 feet x 400 feet turning basin, 8 feet deep. From Western Avenue upstream to the Washington Street Bridge, the channel is now 50 feet wide and 4 feet deep. Dredging is a continuing project. The San Francisco District of the ACOE maintains the San Pablo Bay Channel on a 144-month cycle and the Upper River channel on a 48-month cycle. The Corps’ dredging experience was also used as the basis for evaluating any impacts a proposed project might have on silt deposition in the Petaluma River. Based on the Corps’ experience over the past 50 years, it appears that an average of 60,000 cubic yards of material is deposited in the river each year.

Until 2000, dredging of the river was done about every four years. However, due to funding problems, the work scheduled for 2000 was not begun until 2002 and completed in 2003. The river has not been dredged since then. Justification to pay for dredging comes from the commercial tonnage of three river-using companies in Petaluma: Dutra Materials, Jerico Products, and Shamrock Materials. Although less of a lifeline than it was a century ago, the river still brings in an estimated $10 million annually to the local economy through commercial and recreational uses. The continued reduction of river traffic over time has resulted in the Army Corps downgrading the economic value of dredging the Petaluma River compared to other waterways.

A long-delayed dredging project on the Petaluma River is scheduled to commence in June 2020. Silt has continued to accumulate for 15 years, while the cost of dredging has increased from approximately $1 per cubic yard to between $25 and $30 per cubic yard. It is the city’s responsibility to find a dredge spoils site for the estimated 250,000 cubic yards of muck to be removed from the riverbed. The city plans to use these dredge spoils for habitat restoration. In the longer term, the city is working to establish an innovative regional partnership with the Sonoma County Water Agency, Marin County, the City of San Rafael, the Napa Flood Control District and the Army Corps to dredge a series of overlooked channels across the North Bay.

Dredge boat on the Petaluma River

Dredge boat on the Petaluma River

A dredger boat has some unique features. The working tool resembles a huge potato whip or blender 3-4 feet in diameter and many feet long chops up the sediments as it slowly sweeps back-and-fourth. A stream of river water from the river surface is constantly added to dilute the water-spoils mix which is pumped through a flexible pipe system up to three miles long and 24 inches in diameter to deliver the sludge to the Shollenberger central pond. At this point, the sludge is about 10 – 15 percent sediment, and the rest is river water. Birds gather around the dredge outfall because it is very rich in fish and invertebrates which they eat immediately and for many months following. Mammals such as river otters, muskrats, raccoons and skunks may also join in the feast. The scraps from this feast quickly degrade to detritus, which enriches the water to stimulate plant and invertebrate growth. Dredging is usually done in autumn to protect migratory fishes. If the pond becomes too full, some of the water will be skimmed (decanted) from the surface after the sediment has settled. This decanted water is released through water gates into Adobe Creek, which returns it to the river.

Written by John Shribbs, 2019 based on PWA website article by Gerald Moore, 2012. Pictures courtesy of Gerald Moore.

Bibliography and Resources for more information:

Baumgarten, S.; Clark, E.; Dusterhoff, S.; Grossinger, R. M.; Askevold, R. A. 2018. “Petaluma Valley Historical Hydrology and Ecology Study”. SFEI Contribution No. 861. San Francisco Estuary Institute: Richmond, CA. Available online at:

Yousef Baig. “No Petaluma River dredging next year”. ARGUS-COURIER. December 10, 2018.

City of Petaluma. “Petaluma Watershed”.

Southern Sonoma County Resource Conservation District. “Petaluma Watershed Enhancement Plan”. July 1999.Available online:

The Intergovernmental Panel on Climate Change. “Global Warming of 1.5°C. 2018”. Available online at: Ridolfi, Lester McKee, and Sarah Pearce. Aquatic Science Center. “Petaluma River Impairment Assessment for Nutrients, Sediment/Siltation, and Pathogens”. 2010. Available online:

California State Coastal Conservancy: Oakland, CA. “The Baylands and Climate Change: What We Can Do. Baylands Ecosystem Habitat Goals Science Update 2015”. Available online at:

William J. Mitsch and James G. Gosselink. “Wetlands”, 3rd Edition. 2000.

Climate Central. “Sea level rise threatens plant life in estuaries and salt marshes”, 2018. Available online at: M. Thorne, John Y. Takekawa, and Deborah L. Elliott-Fisk (2012) “Ecological Effects of Climate Change on Salt Marsh Wildlife: A Case Study from a Highly Urbanized Estuary”. Journal of Coastal Research: Volume 28, Issue 6: pp. 1477 – 1487. Available online:

Laffoley, D. & Baxter, J. M. (editors). 2016. “Explaining ocean warming: Causes, scale, effects and consequences”. Full report. Gland, Switzerland: IUCN. 456 pp. Available online at:

“Petaluma River, Detailed Project Report for Flood Control, Sonoma County: Environmental Impact Statement”. 1994. Available online:

Pin It on Pinterest