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Scientist Cautions Against Widespread Dredging in the Delta

July 2011

According to one of the state’s leading geologists, dredging every channel of the Sacramento-San Joaquin Delta is probably not the best idea, though there are some areas where it could be appropriate.

UC Davis Geology professor and Delta Independent Science Board member Dr. Jeff Mount explained his thinking to the Delta Stewardship Council during a recent meeting.

Dredging was not formally part of the agenda, but Council member Randy Fiorini took the opportunity to raise the issue with Mount.

Fiorini noted that the Council had received numerous public comments lauding the positive aspects of dredging; most notable were assertions that it could ensure water supply reliability and reduce the risk of flooding.

Mount agreed that there are benefits to dredging in certain areas of the Delta, namely improving and enlarging channels for ship transportation and higher flood flows.

“At local places in the Delta there are good, rational reasons to get in there and do some dredging,” he said. “But dredging is not a cure-all.”

Decreased water quality due to salt water intrusion, decreased turbidity (clearer water) resulting from sediment removal which would upset the ecosystem, and enormous costs were among reasons Mount listed to avoid dredging the entire estuary.

“As you enlarge the channels, particularly in the western Delta, you are opening up a larger volume of water to the tides,” Mount said. “The last thing you want to see is the western Delta getting saltier.”

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Another risk inherent with additional dredging in the Delta is that sediment removed from one part of the system can taint the water in other parts of the system.

“There’s a reason it’s so difficult to get permits for dredging; there are serious water quality concerns. It’s not like you just pull the sediment out, hermetically-sealed, and put it on the land,” Mount said. “Right now, the only technique for dredging liberates all the loose sediment into the water. Do you wanna drink that?”

As for the claim that dredging would help reduce the risk of flooding by improving existing levees, Mount paused. “We’re never going to (permanently) fix the levees. We’re always going to be adapting…it’s hopelessly naïve to imagine there’s a fix for the levees. We’re going to be working on them for the rest of forever,” he said.

Dredging in the Sacramento Delta began in the 1800s as an inexpensive way for farmers and landowners to raise levees and build up Delta islands quickly. Dredging is used to deepen or maintain navigable waterways and channels that can get filled with sedimented sand, mud and debris over time.

Coequal goals

The Delta Stewardship Council was created in legislation to achieve the state mandated coequal goals for the Delta. "'Coequal goals' means the two goals of providing a more reliable water supply for California and protecting, restoring, and enhancing the Delta ecosystem. The coequal goals shall be achieved in a manner that protects and enhances the unique cultural, recreational, natural resource, and agricultural values of the Delta as an evolving place." (CA Water Code §85054)