Everything You Ever Wanted to Know About Lake Washington

July 30, 2023 | by Naomi Tomky

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As residents of the jewel of Lake Washington, islanders generally know a fair amount about the state’s second-largest lake. But the waters surrounding Mercer Island hold far more history, treasure, and lore than seems possible – from its long history as the center of the region’s transportation, to the planes, trains, and submerged forests that lurk in its murky depths.

Formative Years

The Vashon ice sheet moved through the area in the Late Pleistocene era and created the 18-mile long, three-mile-wide ribbon lake, fed by the Cedar River at the southern end and the Sammamish River further north. While it used to drain into the Black River, its main outlet is now the Lake Washington Ship Canal.

But nature wasn’t done with the lake quite yet: about a thousand years ago, a massive earthquake sent huge swaths of hillsides sliding into the lake, the tall trees growing on them and all. In three places – one in Kirkland, and two just off the southern and western coasts of Mercer Island. Though the trees are not actually growing or living underwater, the lack of oxygen preserves them, and since they landed still standing up, it gives the appearance of an underwater forest.

For more than 12,000 years, the lakeshore served as home for many of the local Indigenous people of the region. Some of their words stick around as part of place names along the coast, including Enatai, a Chinook Jargon term meaning “the other side.” Other words faded away, such as Hyas Chuck and Xacuabš, Chinook Jargon and Duwamish words meaning “big water,” respectively – the latter referring to the Duwamish themselves as the people of that lake. Some state maps even referred to the lake as Lake Duwamish for a time, before Thomas Mercer suggested it be called Lake Washington in 1854 – at the same party his friends suggested Mercer Island take his name (see more in our article on Mercer Island place names).

He also suggested the name for Lake Union, a hint at the future of the lakes and their connection to Puget Sound. Construction on the Lake Washington Ship Canal, connecting freshwater to saltwater didn’t begin for about five more decades, when Hiram Chittenden proposed the building of the locks, solving the issues of the disparate water levels. The locks were finished in 1916 and the canal officially opened a year later.

The building of the canal lowered Lake Washington by nine feet, opening up new waterfront property and moving the borders of the surrounding wetlands. It dried up the Black River, near Renton, rerouting the lake’s outlet to the canal and obsoleting the river’s main use for settlers by creating a more direct route for boats into the lake.

What Lurks Below

Not that the difficulty stopped anyone from bringing boats the long way around: steamboats and barges plied the lake from the late 19th century, built on the lake or dragged up the Black River. They transported both people and products, including coal and timber from the Eastside foothills, two very important industries at the time. Things didn’t always go smoothly though. During a winter storm in 1875, 12 coal cars tumbled off of a barge transporting Newcastle’s vast and valuable resource across the lake and they joined the underwater forests on the lake bottom.

The coal cars remain just south of the 520 bridge, but while they were the first wreck to sink, they were far from the last. Ferry services and pleasure cruises joined the traffic on the busy lake and formed a main mode of transportation in the pre- and early-automobile era, and like the coal cars, some 400 boats still make the 22,000 acres Lake Washington their home. They include ferries, three Navy minesweepers, and a litany of other random private boats that when people decided they didn’t want them anymore, they simply found a way to sink them. There’s also a huge World War II-era bomber plane, plus at least six other planes lost during mock battles over the lake.

Pictured above: the steamboat Dawn, a passenger ferry that provided public transportation between Seattle, Mercer Island, and Bellevue, from 1914 to 1938 – prior to the construction of the I-90 floating bridge. The steamboat now rests at the bottom of Lake Washington off the southern shore of Mercer Island.

Cleaning Up

It’s now illegal to sink (or “scuttle) a boat or any kind of vehicle in a Washington lake or waterway because of detrimental impacts to the environment from pollution from fuel and other harmful contaminants, in addition to risks to human health and safety. In 2002, the Washington State Department of Natural Resources instituted the Derelict Vessel Removal Program to address the problem of sunken boats that pose risks to the environment and human safety. (Though few that reside on the bottom of Lake Washington have been noted to be of concern).

Sunken vessels haven’t been the only environmental concern for Lake Washington, however. From 1941 to 1963, the growing city of Seattle let secondary-treated sewage flow into the lake, causing eutrophication (an increase in minerals and nutrients) and destroying the water quality. Blue-green bacteria (algae) grew because of the phosphorus in the treatment, and eventually made the water darker, less welcoming to fish populations, and, when it washed ashore, the beaches stinkier.

Metro formed in 1958, bringing together all the municipalities along Lake Washington, to combat the issue of dirty lake water. The solution was a $140 million sewage diversion project that started in 1963 and fixed the problem by 1968. The lake has since remained mostly clean and safely swimmable.

Wildlife in the Lake

The excess phosphorus changed what could live in the lake’s water, but so too have other human changes, including introduction of non-native species – like the famous nuisance of milfoil, introduced in the 1970s. About 40 species of fish live in the lake, including such fun-named creatures as three-spine sticklebacks, peamouth chub, and black crappies. Coho, Chinook, and sockeye salmon make their way through the lake, and their freshwater trout brethren also swim around, along with the coastal cutthroat and Kokanee. Multiple kinds of turtles, signal crayfish, and freshwater mussels and clams also make their home in and around the lake.

Today’s Lake

Thanks to the environmental improvements and new protections over the decades, the lake has been restored as a delightful home to plenty of creatures. For local humans, that makes the lake a refreshing part of living in the area, a cool, clear playground for swimmers and sailors. To learn more about where to enjoy the lake on Mercer Island, see our Guide to Mercer Island beaches. And, while there is nowhere to rent a boat or board on the island, we’ve rounded up the best places nearby to rent kayaks, paddleboards, canoes, jet skis, and more.