You’ve probably seen it from the jagged top of Rattlesnake Ledge. That long, shimmering stretch of blue tucked deep into the Cascades, looking like the ultimate spot for a summer kayak session or a quiet afternoon of fishing. It looks perfect. It looks inviting.
It’s also completely illegal to touch.
Most people staring down from the cliffs at Chester Morse Lake WA assume it’s just another recreation spot like Lake Washington or Lake Sammamish. Honestly, that’s the first thing everyone gets wrong. If you try to drive up to the shoreline for a swim, you’re going to run into a very serious gate and probably a very unhappy ranger.
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This isn't just a lake; it's a giant, 1,600-acre "keep out" sign. And for good reason.
The Hidden Engine of Seattle
Basically, Chester Morse Lake is the primary "gas tank" for Seattle’s water. About 70% of the drinking water for the greater Seattle area comes from this exact spot. Because of that, the entire Cedar River Municipal Watershed is a closed-access zone.
No swimming. No dogs. No hiking (mostly). No motorized boats.
It’s a weirdly pristine bubble in a state where almost every other mountain lake is crawling with influencers and paddleboards. Because the city has owned this land since the early 1900s, it hasn't been developed. There are no lakefront condos or "rustic" Airbnbs. It’s just old-growth forest and water so clean that Seattle is one of the few major cities in the U.S. that doesn't have to filter its water. They just treat it and send it to your tap.
A Disaster That Swallowed a Town
Here is a bit of history that usually gets skipped over. The lake wasn't always this big. It used to be a natural body of water called Cedar Lake, formed about 14,000 years ago by a glacier.
Around 1900, the city decided they needed more water and more power. They built a timber dam, and later, the massive Masonry Dam. They wanted to raise the water level by over 50 feet.
It was a total mess.
The engineers didn't realize they were building on a glacial moraine—basically a giant pile of loose gravel and dirt left behind by the ice age. As the water rose, it didn't just stay in the reservoir. It started leaking through the hillside like a sieve.
The most dramatic part? The "leak" actually flooded the nearby town of Moncton. Residents literally watched as water bubbled up through the floors of their homes. Eventually, the entire town was submerged and condemned. If you go to Rattlesnake Lake today (which is right next door and is open to the public), you’re actually looking at the byproduct of that seepage.
The Secret Wildlife Vault
Since people aren't allowed in, the area around Chester Morse Lake WA has become a de facto nature preserve. It’s actually kind of cool—and slightly creepy—how much stuff lives there that we never see.
- Bull Trout & Pygmy Whitefish: These fish have been isolated in the lake for over 13,000 years. Because of the Cedar Falls waterfall downstream, they’re trapped in a genetic time capsule.
- Common Loons: Only a few pairs of these birds nest in Washington, and Chester Morse is one of their few reliable havens.
- The Big Guys: There are confirmed sightings of grizzly bears, gray wolves, and wolverines in the upper watershed.
It's one of the few places in King County where a wolf could actually wander around without running into a mountain biker.
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Can You Actually Go There?
Sorta. But you have to be strategic.
You can’t just go. You have to book a tour through the Cedar River Watershed Education Center. They run these tiny, limited-capacity van tours during the summer. It’s the only way to legally cross the Masonry Dam and see the lake from the shore.
If you can’t snag a tour, the best "legal" view is still the Rattlesnake Ledge trail. Just remember: when you're looking down at that water, you're looking at what you'll probably be drinking tomorrow morning.
What You Should Do Next
If you're planning a trip to the area, don't just show up at the watershed gates. Instead, start at the Cedar River Watershed Education Center near North Bend. It’s free, the exhibits are actually decent, and they have "rain drums" that play music based on the weather.
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- Check the Tour Schedule: The 2.5-hour "Heritage Tour" is the one that actually takes you to the lake and the dam. These sell out weeks in advance.
- Visit Rattlesnake Lake: This is the "overflow" lake where you can swim and kayak. It’s right outside the restricted zone.
- Hike the Ledge: For the big aerial view of the reservoir, hit the Rattlesnake Ledge trail early (before 8 AM) to avoid the crowds.
Respect the boundaries. The sensors and patrols in the watershed aren't just for show—they’re protecting the water for nearly two million people.