Advisory Council: Dredging needed to prevent Silver Lake from becoming a marsh

In this August 2018 photo, a warning sign posted by the Cowlitz County Health and Human Services Department at Streeter's Resort on Silver Lake states toxic algae is making the water unsafe for people and pets.

Posted on May 11, 2021

CASTLE ROCK — Silver Lake, a popular bass fishing spot, is on track to become unsuitable for any fish populations of recreational or economic value, according to the Silver Lake Watershed Advisory Council.

“Put simply, the lake is dying,” said Silver Lake resident and Advisory Council member Del Gilkerson during a presentation to Cowlitz County commissioners. “Without intervention it will turn into a marsh.”

The 1,650-acre lake east of Castle Rock has unhealthy levels of nutrients that cause dense plant growth, animals deaths and toxic algae, according to the advisory council.

Representatives from the council pitched several solutions to county commissioners late last month but said partial dredging is the only long-term solution.

Silver Lake used to flood in the winter and flush in the summer, creating an exchange of freshwater, Gilkerson said.

In the 1970s, Outlet Creek was widened to give floodwater a place to go and a weir was built so the lake wouldn’t drain out. That decreased the exchange of freshwater, leaving the lake stagnant in the summer, he said.

A 1990 study found the lake is in an advanced stage of eutrophication, meaning excessive nutrients are causing dense plant growth and killing animals that rely on oxygen in the water as it becomes depleted.

The study included several restoration recommendations, including plant control, waterfowl management, bottom screening, watershed management and septic tank controls, but stated dredging is the only long-term solution.

In 1992, about 83,000 grass carp were introduced to the lake for plant control, Gilkerson said. The carp controlled the milfoil but also ate all the beneficial vegetation, he said.

To reduce the number of grass carp, the advisory council helped push for Washington Department of Fish and Wildlife approval to fish them out, Gilkerson said.

“They’re still in there, there’s not many,” he said. “People that live along the lake they’ve told me they see fewer and fewer grass carp. So, I think eventually they’re going to be gone.”

Increased amounts of toxic algae have been found in the lake since the 2000s, including cyanobacteria, also known as blue-green algae. The algae thrive in warm, nutrient dense water and can be harmful to people and animals.

The Silver Lake Flood Control District lowered the lake by three feet in 2018, attempting to flush and refill it with fresh water in the winter. Gilkerson said the lake took longer to refill than expected and the move didn’t have the effect they hoped.

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