Good news for Green Lake: It's clean and staying that way. Efforts in 2004 to reduce algae growth in the lake by adding aluminum sulfate...

Share story

Good news for Green Lake: It’s clean and staying that way.

Efforts in 2004 to reduce algae growth in the lake by adding aluminum sulfate, or alum, are still paying dividends. The alum binds with phosphorus in the water, tying up a key nutrient algae needs to thrive.

Three years of monitoring the lake show that improvements in water quality are holding steady, according to findings presented Tuesday by King County staff and the Friends of Green Lake, a nonprofit citizens group.

Green Lake is very old and much modified. Its levels have been lowered and plumbing added to manage those levels. And the lake needs to be maintained by people to stay clean and sparkling for swimming and boating.

This week, save 90% on digital access.

Not for nothing is it called Green Lake. Algae growth is natural, and some amount of algae is healthy. A lake with no algae would be a dead lake. But the addition of nitrogen and phosphorus from fertilizers and other pollutants, combined with warm, sunny days, can spell excessive growth.

And blue-green algae, while quite beautiful, are toxic to small animals and dogs, said Tricia Shoblom of the state Department of Ecology. The department works with local health districts to monitor algae blooms in lakes.

The alum treatment, while effective against algae, does have a downside. With sparkling clear water, more light reaches the lake bottom, which encourages the growth of milfoil. An invasive exotic, milfoil breaks into tiny pieces that root and grow into new plants.

New colonies of milfoil have been sprouting in the lake lately and may become problematic. The city has even had to mow it in years past.

But so far, Sally Abella, program manager for the lakes stewardship program at King County, said she is encouraged with the trends at Green Lake.

Green Lake had to be closed to swimming in 2003 because of water-quality problems. But now chlorophyll, phosphorous, nitrogen, and clarity measures all look good. “We are not seeing a return to previous conditions,” Abella said.

Also, a 95,000-gallon sewage spill last December, caused by a heavy rainstorm, did not harm the lake, Abella said, because the lake is large enough to dilute the problem. The lake also mixes rapidly as wind rakes across it, churning its shallow water.

The 268-acre lake is smaller than Lake Washington, Sammamish or Union, but no lake in the area is more popular. As the weather warms and days lengthen, Green Lake is ready to receive a grateful public.

“It’s in great shape,” Abella said.

Lynda V. Mapes: 206-464-2736 or

Custom-curated news highlights, delivered weekday mornings.