What does Lake Washington’s warming mean for its future?

The cold, watery heart of the region lies between Seattle and the Eastside. It uniquely supports two major roadways over floating bridges, has provided beachgoers with summer respite for decades, and is central to the Seattle area’s cultural identity.

But Lake Washington has been changing by half a degree Fahrenheit every recent decade. In fact, since 1963, lake levels have warmed about 4.3 degrees from June to September, according to data collected and analyzed by King County and the University of Washington.

While some of the lake’s variability can be attributed to natural, long-term climate variability in the Pacific Ocean, Global climate change Greenhouse gas emissions are definitely contributing to warming the lake, said King County water quality engineer Curtis DeGasperi, who manages the lake’s monitoring program.

De Gasperi said the lake warmed earlier in the year and took longer to cool in the fall and winter months, which also showed a slight warming trend.

“It’s not going to be the same lake. It’s going to change, and people need to sit around and think about it to anticipate and prepare for that,” he said. ” “We know, it’s definitely going to be hot. There’s no doubt about it.”

It’s unclear exactly what the warming trend will mean for the hundreds of people who flock to Lake Washington each summer for easy access to swimming and boating. Even with the increase in the region’s population, the lake has become cleaner. Along with wastewater infrastructure, the nutrients that feed algae, which can cause blooms and adverse conditions, have declined over the past two decades, bucking the trend seen in most urban waterways.

But warmer waters are certainly harmful to endangered salmon that depend on cool, well-oxygenated water to survive, and Lake Washington has seen more days when its surface water than has risen above what salmon can tolerate.

In each of the past 10 years, the lake has experienced more than 40 days each summer when its surface has exceeded 68 degrees Fahrenheit. That compares with the only three years of warmer temperatures between 1960 and 1980, according to an analysis by the county and the University of Washington.

They Surface water That’s what flows into the Ship Canal, which connects the lake to Puget Sound through the Ballard Locks, DeGasperi said. During the summer and fall months, that warm water in the commercial and recreational channel, about 30 feet deep, becomes a migration barrier for adults. Juvenile salmon trying to pass—either looking for a chance to reproduce upstream or swimming to their adult life in the ocean.

What did it look like?

Lake Washington wasn’t always what it is today.

Between the 1940s and 1960s, more than 20 billion gallons of untreated sewage flowed freely into the lake each year. Daniel Niedzgorski, a King County ecologist, said people could barely see a few feet into the water, beaches were often closed and algae blooms had given the lake a “reddish” color.

The lake experienced a near-miraculous turnaround after King County built two wastewater treatment plants—one in Discovery Park and the other in Renton.

By the 1980s, pollution spills into the lake had shrunk to 2.3 billion gallons a year, and last year, 1.7 billion gallons of untreated stormwater and sewage flowed into Puget Sound and Lake Washington. Ongoing projects aim to further reduce this amount.

Lake Washington is a success story equal parts luck and municipal planning, Nidzgorski said.

Investments in stormwater infrastructure have paid off, and now decades later the water is cleaner and cleaner than it has ever been in Seattle’s modern history. During a round of budget cuts in 2009, the county also stopped analyzing its samples for bacteria, concluding that the levels weren’t changing enough to be measured, he said.

“What we’re doing is actually working,” Niedzgorski said. “It’s really good news that we’ve adopted a lot of new regulations, better technology, just better practices.”

But that doesn’t mean there won’t be challenges for the future. Longer summers mean that the period of lake stratification — when the water column forms separate layers that barely mix — will be longer. Currently this has been primarily a problem in Lake Sammamish, where kokanee salmon and their predators are forced into a narrow band of breathable water between a warmer surface and a lower layer with less oxygen.

However, a potential risk of prolonged stabilization periods for all lakes is the potential to affect non-toxic and toxic algae blooms later in the fall, DeGaspari said. When the bottom layer of a lake loses oxygen during the summer, it can trigger the release of phosphorus from the sediments. That phosphorus — which can support algae growth — later dissolves into the lake when fall temperatures arrive.

Future Challenges for Ballard Locks

There is a pinch point at Ballard Locks and the wider Ship Canal, which runs through Salmon. Before the locks were built, and the entire water system was dammed in the 1910s, Lake Washington was 8 feet high and its annual elevation fluctuated with mountain flow.

Now, the folks at Ballard Locks carefully engineer the lake’s surface.

As the lake is expected to warm with climate change, government officials and engineers are exploring plans to cool the canal so salmon can still rely on it for migration during the warmest months of the year.

For out-migrating salmon and juveniles, the only places with reliably cool, well-oxygenated water are at each end of the 7-mile canal: in a chilly Puget Sound, just west of Ballard Locks, where opportunity Pristine seals camp. , and another in Lake Washington, where the water drops more than 200 feet in its deepest parts and warm-blooded fish like yellow perch and bass have increased appetites as the water has warmed.

Lauren Urgenson, former King County coordinator for the Lake Washington, Sammamish and Cedar watersheds, said the sudden change from oxygen-rich saltwater to warm freshwater is a shock to salmon in the locks. Adult Chinook salmon often cycle through the fish ladder multiple times or wait upstream for weeks, losing energy and vulnerable to predator attacks, he said.

Whenever the locks are close to ferry kayakers and boaters through an engineered dam, the salmon get a breath of cool, oxygen-rich water. In the past, engineers have experimented with “false locks,” or opening and closing the locks even when boats aren’t around, Argenson said, but they’ve found that this pushes oxygen only a short distance and Not for long.

Whenever the locks are operated, the freshwater upstream side of the lock loses water. Drought and reduced river flows due to climate change are expected to affect work on the locks, said Kyle Comnor, senior water manager for the Army Corps of Engineers’ Seattle District. The lock is Lake Washington’s only outlet, he said, and engineers must keep the lake’s water level stable and not allow too much saltwater to leach inland. In addition to management options, boater wait times at the lock may be increased or scheduled to conserve water in the future.

Meanwhile, mature sockeye salmon have been trucked around the canal and Lake to increase their survival. During and after the record-breaking heat wave of 2021, dead salmon were observed in the fish ladder of locks. While salmon won’t go into water warmer than 70 degrees, any water above 59 degrees is considered “sublethal,” which stresses the fish and makes them susceptible to disease and growth problems.

Tribes, federal and local, to receive cooling water along the entire length of the Ship Canal Government officials And members of Long Live the Kings, a Seattle nonprofit dedicated to salmon recovery, have proposed and are evaluating solutions.

These ideas include various methods of pumping cold water from the depths of Lake Washington directly into the ship’s canal with a series of pipes and valves, or using a heat exchanger to cool the water in the canal.

“If we want salmon here — and salmon have done so much for our region and for us — we need to solve this problem,” Argensen said. “Seattle without salmon is not a great future.”

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