Mechanical harvester clearing invasive weed off Spring Lake

A bright blue machine that resembles a cross between a bulldozer and side-wheel paddleboat is busy cleaning up a big mess Mother Nature left this year at Spring Lake.

The 32-foot-long aquatic harvester operated by a Windsor company is traversing the lake, collecting more than a ton of a floating weed called Azolla in its 8-foot-wide steel mouth on each foray over the 72-acre recreational lake.

At the Spring Lake boat dock, the harvester’s stern transfers the soggy plant mass onto an up-tilted conveyor belt that drops it into a red trailer for delivery to county property at Los Guilicos, where the weed will be mixed with other organic material to make compost.

It’s a new approach to a familiar problem on the lake managed by the Sonoma County Regional Parks, as an unprecedented bloom of Azolla spread a harmless but unsightly reddish-brown carpet over large portions of the lake, popular with boaters and anglers.

“We’ve never seen it like this,” parks manager David Robinson said. “We haven’t had Azolla this bad.”

With the upcoming busy Memorial Day weekend, officials decided on a new type of cleanup, calling on the harvester from Windsor-based Waterworks Industries, a statewide firm whose services include mechanical vegetation removal.

In past years, parks officials resorted to herbicide spraying to get rid of invasive weeds, Robinson said. The drawback is that dead plants sink to the bottom, adding to the lake’s nutrient load that can nourish blue-green algae, which can release harmful toxins.

The diesel-powered harvester, which started work Monday and could continue through next week, is more like a vacuum cleaner leaving little behind in its wake.

The county is paying Waterworks Industries about $3,000 a day for the cleanup, with a total cost not to exceed $32,000, Robinson said.

“It looks leaps and bounds better than it did on Monday,” Robinson said. “We really feel comfortable this is the right method.”

Paul Meehan, who bought a home near Spring Lake so he could kayak frequently, said it’s made a big difference.

Azolla “made it almost impossible to enjoy” the experience, he said.

“It’s like trying to paddle through oatmeal,” Meehan said. “It clings to the boat; it clings to the paddle.”

Azolla, also known as water fern, commonly grows on small lakes and reservoirs and is “pretty innocuous,” a state water board official said last month.

Other experts suggested the abundant bloom this spring may have been fertilized by fire retardant dropped nearby during the October wildfires leaching into the lake. Cal Fire released about 2 million gallons of retardant in an effort to slow the flames that charred 137 square miles of Sonoma County.

A Cal Fire official, however, said there was “nothing to substantiate” such a connection.

The lake was dredged more than 20 years ago, and the spoils were deposited behind the horse trailer parking lot near the park’s Newanga Avenue entrance, Robinson said. The mound is now overgrown and resembles a small hill.

Dredging left the lake 10 to 15 feet deep but due to subsequent flooding and sediment-laden runoff from Santa Rosa Creek it’s now mostly?5 to 7 feet deep.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com. On Twitter @guykovner.