Dredge Lake Mendocino? Experts say it’s ‘not worth the bang for the buck’

Now that Lake Mendocino holds only a fraction of what it should, there’s a whole lot of lake bed exposed to the sky, and folks are wondering why the government doesn’t exploit the opportunity to dredge the lake and increase its capacity for the future.

It’s an increasingly common question as the lake has receded from the shoreline over months of severe drought, and the prospect of increasing extremes in rainfall going forward accentuates the need for expanded water storage.

But it’s not as obvious a solution as it may seem, officials say.

Though a substantial volume of sediment has accumulated in the lake bottom since the reservoir outside Ukiah was dammed and filled more than 60 years ago, the cost and trouble of dredging it suggests a “bang for that buck that is pretty low,” one U.S. Army Corps of Engineers official said.

Estimates suggest the hardened silt now occupies the space that would otherwise store between 7,500 and 8,500 acre feet of water — or almost 14 million cubic yards.

That’s equal to about 7% of the lake’s overall storage capacity of 118,000 acre feet, or about 64% of what the diminished reservoir currently holds. (The lake held a little more than 13,000 acre feet on Friday, or about 4.2 billion gallons of water.) It’s not an enormous amount of space, but having it available for surplus water in the future would be welcomed.

But removing the sediment would be a great big deal, officials said.

“The volume is staggering,” said Brad Sherwood, a spokesman for Sonoma Water, which manages lake releases in partnership with the U.S. Army Corps of Engineers. “If you piled the sediment at the bottom of Lake Mendocino on to one football field, it would be almost a mile and a half high, or over a million truck loads.”

Even if you moved 100 truckloads a day of earth excavated from the bottom of the lake, it would take years — assuming the lake stayed dry enough to work that long, he said.

But even before it got to that point, tapping the millions of dollars in federal funds and acquiring authorizations necessary to do the work would require studies and environmental assessments, not least because of the turbidity that would be churned up, potentially affecting imperiled fish downstream, said Townsley, with the Army Corps.

There also would be important questions about emissions and air quality related to trucking away all that earth.

And even though the amount of sediment in the lake seems like a lot, it’s relatively little compared to reservoirs in steeper watersheds where water flows in at a higher velocity, bringing in more soil and silt with it, Townsley said.

Then there are the questions about the mud, once it’s dredged, he said.

“What are you going to do with it? Where does it go — even assuming you’re not going to find too much bad stuff in there?”

“It would be, I think, a lot of effort for not very much return,” Townsley said.

“We do, in a moment like this, understand that intuitive thought that, like, ‘Hey, let’s dig it out,’ ” said Nick Malasavage, operations and readiness chief for the Army Corps’ San Francisco District. “ … For us, the big takeaway is that every scoop of earth is a whole lot of investment in permits and energy to get it done.”

Townsley said that doesn’t mean the time for dredging would never be right.

“At some stage of the game, the water may in fact become valuable enough that we, as a society, start making decisions to do stuff like that,” he said. “I just don’t think we’re there yet.”

You can reach Staff Writer Mary Callahan at 707-521-5249 or mary.callahan@pressdemocrat.com. On Twitter @MaryCallahanB.