California WaterBlog

By Madison Dunlap, Aidan Rafols, and John Durand

. . .

Overbite clams (Potamocorbula amurensis) are a small species of bivalve native to Eastern Asia (Figure 1). They have been present in the San Francisco Bay estuary since 1986, and given their ability to quickly multiply and spread, are considered highly invasive within this range (Carlton et al., 1990). Overbite clams consume phytoplankton and copepods, which are also necessary food for the early life stages of many native fish. The sheer abundance of these overbite clams has contributed to the collapse of the pelagic food web in Suisun Bay due to loss of those prey species (Sommer 2007). This has done extensive damage to local populations of many iconic fish species like the Delta smelt (Figure 2), longfin smelt, and striped bass (Sommre 2007). 

Despite this sad state of affairs, there may be some hope on the horizon to rein in the destructive impacts of overbite clam. Researchers at the Center for Watershed Sciences at the University of California, Davis have been monitoring the Suisun Marsh for the past 40 years. The marsh, the most extensive remaining wetland in the San Francisco Estuary, is a hotspot for native fishes (Figure 3; Stompe 2023). Using data from the Suisun Marsh Fish Study, researchers have found that the population of adult reproducing overbite clams in Susiun Marsh is spatially limited to a single large slough (Suisun slough; Baumsteiger 2017). While the exact cause for their residency in this slough is unknown, it is hypothesised that some combination of abiotic and biotic factors cause lower survival rate in interior channels and prevent Overbite clams from establishing in smaller sloughs (Baumsteiger 2017). Effective mitigation of overbite clams — at least in Suisun Marsh — could be possible if this one source population was effectively managed. Additionally, targeting a single spatially limited population is much more feasible than spreading out mitigation efforts, but tackling this problem is still no easy task. 

Proposal: When I was approached by John Durand, a Senior Researcher at the Center for Watershed Sciences, with the proposition of investigating how feasible it would be to remove the overbite clams from Suisun Slough, I had the same question everyone else did: how? It turns out this somewhat out-of-the-box thinker had a solution in mind — just remove the substrate with the clams, leave it to dry (long enough for the clams to dessicate), and then put the sediment back, dead clams and all. In essence, just dredge it up! 

Would it work? Some research has shown that bivalve populations can be controlled via dredging, such as Sheehan et al. 2014, which utilized commercial clam fishery dredges to capture and remove Corbicula — another small free-living invasive clam — from a shallow tidal system in Ireland. Witman et al. 2012 also found success in Corbicula mitigation by using suction dredges in the shallow areas of Lake Tahoe.

While both previous studies found success in small and/or shallow systems, their respective dredging techniques may not be as effective in large systems like Suisun Slough, which is up to 40 ft deep and 120 ft wide.This system will likely require much larger infrastructure to effectively dredge its entirety, but the successes of these previous works are enough to make me think this is a potentially winning strategy. All that stands in the way now is finding the correct dredging strategy for Suisun Slough.

What kind of dredge? Luckily for us, the San Francisco Bay is no stranger to large-scale dredging projects. The Sacramento Deep Water Ship channel, a large shipping channel that runs to the Port of Sacramento, has been dredged annually to prevent substrate infilling since 2022. In addition, a recent initiative, called the South Delta Channel Depth Restoration Program, plans to improve flows from the San Joaquin/Sacramento river systems by dredging over 75 miles of the South Delta. These projects are at a similar scale to the Suisun Slough dredging we propose here, and the types of dredges used (clamshell and/or bucket dredges; Figure 4) would be sufficient for the project. 

How much would it cost? Short of a complete environmental assessment, there is no perfect way to estimate the cost of a project like this. Suisun Delta’s main channel is 13 mi (20921.5 m) by 120 ft wide with a max depth of 40 ft. If we assume we have to dredge about 16 in of sediment (roughly the portion of sediment that overbite clams utilize) we get 123,979.26 yd³ in total.

One way we can estimate cost is by looking at the cost of other local projects. Dredging the 43 mile long Sacramento Deep Water Ship Channel costs the state about $7 million a year (US Army Corps of Engineers 2024), while the South Delta Channel Depth Restoration Program is estimated to cost about $10.6 million for every 100,000 yd³ (State Water Contractors 2022). If we scale the cost of these projects to the dredging plan proposed here, the estimated cost to dredge Suisun Slough would be, very roughly, between 2.1 and 13.4 million dollars. 

But to obtain a more accurate cost estimation for dredging Suisun Slough, one would have to consider the unique needs of the project. If we were to do a more robust cost estimation, the following information would likely need to be considered:

• This is a deep water dredging project which increases cost.
• The cost for dealing with sediment will be much lower because we are returning it to site.
• Dredging for this project would best be achieved through mechanical dredging, which is common in San Francisco Bay so the infrastructure is already present.
• And of course, cost will scale with the area dredged, so the exact extent of the dredging operation would be necessary to calculate cost.

The conclusion is that the costs estimated here are fairly low considering the potential benefit. The state’s main strategy to mitigate this issue has been to restore fish habitat, a costly solution, or to divert water, a politically divisive move. California has spent well over $200 million dollars to enhance habitat for fish since the early 2000s (Johnson 2008, California Department of Water Resources 2024), and diverting water from the central valley for environmental reasons has historically produced extensive backlash. Though these conservation efforts undoubtedly improve environmental outcomes, species like the Delta Smelt are still in decline. While dredging Suisun Slough won’t entirely eliminate overbite clams, knocking down the reproductive population may provide an opportunity for other less-detrimental benthic species to establish in Suisun Slough. So at a time when all prior solutions have been ineffective or unpopular, why not try adding something new?

About the Authors

Mady Dunlap is a Junior Specialist in the Aquatic Research Collective Lab where she primarily works to support the North Delta long-term monitoring project. She has a background in bivalve ecology, previously working with native and invasive bivalves in the Great Lakes region.

Aidan Rafols is a student assistant in the Aquatic Research Collective Lab. He conducts zooplankton analysis for fish food-web studies in the North Delta and is currently conducting a fish assemblage analysis of lower Cache Creek. He is majoring in Wildlife Biology and minoring in English.  

John Durand is a senior research scientist at the UC Davis Center for Watershed Sciences. 

Further Reading

Baumsteiger, J., R. E. Schroeter, T. O’Rear, J. D. Cook, and P. B. Moyle. (2017) Long-Term Surveys Show Invasive Overbite Clams (Potamocorbula amurensis) are Spatially Limited in Suisun Marsh, California. San Francisco Estuary and Watershed Science 15. https://doi.org/10.15447/sfews.2017v15iss2art6 

California Department of Water Resources.(2024, November 13) Reclamation, DWR Celebrate Launch of Key Delta Habitat Restoration Project. California Department of Natural Resources, News Releases. https://water.ca.gov/News/News-Releases/2024/Nov-24/Reclamation-DWR-Celebrate-Launch-of-Key-Delta-Habitat-Restoration-Project

Carlton, J., J. Tompson, L. Schemel, and F. Nichols. (1990) Remarkable invasion of San Francisco Bay (California, USA), by the Asian clam Potamocorbula amurensis. I. Introduction and dispersal. Marine Ecology Progress Series 66, 81–94. https://www.jstor.org/stable/24844648

Johnson, R. W. (2008, January 29) California Water Issues. Statement of Robert W. Johnson, Commissioner Bureau of Reclamation U.S. Department of the Interior Before the House Natural Resources Committee. Subcommittee on Water and Power. U.S. Department of the Interior. https://www.doi.gov/ocl/hearings/110/DeltaSmelt_012908 

Sheehan, R., J. Caffrey, M. Millane, P. McLoone, H. Moran, and F. Lucy. (2014) An investigation into the effectiveness of mechanical dredging to remove Corbicula fluminea (Müller, 1774) from test plots in an Irish river system. Management of Biological Invasions 5, 407-418. http://dx.doi.org/10.3391/mbi.2014.5.4.11

Sommer, T., C. Armor, R. Baxter, R. Breuer, L. Brown, M. Chotkowski, S. Culberson, F. Feyrer, M. Gingras, B. Herbold, W. Kimmerer, A. Mueller-Solger, M. Nobriga, and K. Souza. (2007). The Collapse of Pelagic Fishes in the Upper San Francisco Estuary: El Colapso de los Peces Pelagicos en La Cabecera Del Estuario San Francisco. Fisheries 32, 270–277. https://doi.org/10.1577/1548-8446(2007)32[270:TCOPFI]2.0.CO;2

State Water Contractors (2022) South Delta Channel Depth Restoration Program. State Water Contractors. FINAL_Delta-Dredging-Fact-Sheet https://swc.org/wp-content/uploads/2022/03/FINAL_Delta-Dredging-Fact-Sheet.pdf 

Stompe, D. K., P. B. Moyle, K. L. Oken, J. A. Hobbs, J. R. Durand. (2023) A Spatiotemporal History of Key Pelagic Fish Species in the San Francisco Estuary, CA. Estuaries and Coasts 46, 1067–1082. https://doi.org/10.1007/s12237-023-01189-8

Wittmann, M. E., S. Chandra, J. E. Reuter, A. Caires, S. G. Schladow, M. Danton (2012) Harvesting an invasive bivalve in a large natural lake: species recovery and impacts on native benthic macroinvertebrate community structure in Lake Tahoe, USA. Aquatic conservation 22(5), 588-597.  https://doi.org/10.1002/aqc.2251 

US Army Corps of Engineers (2024, February 29) Sacramento River 30 ft Channel (O&M). US Army Corps of Engineers – San Francisco District https://www.spn.usace.army.mil/Missions/Projects-and-Programs/Projects-A-Z/Sacramento-River-30ft-Channel-O-M/