River or Slough? The Sammamish Can't Decide, and Neither Can We

Quick quiz for my fellow Eastsiders: what do you call the flat, slow-moving waterway between Lake Sammamish and Lake Washington that you bike, run, or kayak along every summer?

If you said "river," you're only sort of right. If you said "slough," you're also only sort of right. The U.S. Geological Service (USGS) calls it the Sammamish River. The river supplies 27% of Lake Washington’s water.

Longtime locals still call it the Sammamish Slough. Old maps call it the Squak Slough. Nobody, apparently, felt like settling this before naming an annual hydroplane race after it.

Image by Galeeb Kachra

So which is it?

Technically, a slough is a slow, marshy, meandering channel — which is exactly what this waterway was for most of its existence. It wound through a wide, swampy valley for 30 miles. What you see today, a tidy, 10-to-14-mile, laser-straight ditch with a bike trail on top, is a river only in the sense that a canal is a river. Both run where engineers told them to run.

From Slough to River

Farmers started straightening it in the early 1900s to drain the valley for agriculture. In the mid-1960s, the U.S. Army Corps of Engineers and King County finished the job: a $3.75 million project that took the winding original, lowered the bed by up to 7 feet, and turned "30 miles of squiggles and kinks" into roughly 10 miles of straight, steep-sided channel. They even had to shut down the beloved (and genuinely reckless) Sammamish Slough hydroplane race in 1976 after a spectator got hit. But the straightened channel had already sucked most of the thrill out of it anyway.

This wasn't a rogue engineering project. It's part of a global playbook: Florida channelized the Kissimmee River in the 1960s for flood control, then spent two decades (and hundreds of millions of dollars) trying to put the bends back in. Los Angeles famously entombed its river in concrete. Germany did the same to long stretches of the Rhine in the 19th century for flooding, farmland, and the opinion that nature just needed better lines. Bihar straightened its Kosi River; but a 2008 breach reclaimed the old channel and displaced millions. And Jo’berg’s Juksei River that once served the city’s water demand is now relegated to an underground straight concrete channel. The Sammamish is therefore just another chapter of rivers that we "fixed" in the last century and are now being expensively un-fixed.

The gauntlet: orcas, sea lions, then a ditch

Here's what an adult Sammamish-bound salmon actually has to survive if it makes it back from its ocean journey. First it has to get past the Ballard Locks, where — as I wrote a while back — seals and sea lions have learned that the fish ladder is basically an all-you-can-eat buffet. Once through Lake Washington, it must navigate our straightened, shadeless, current-scoured ditch — with none of the pools, side channels, or overhanging cover that used to slow the water down and hide a tired fish from a hungry bird or warmer temperatures.

Why you should still care about a ditch

A 1950 state report found the Sammamish River once held about half the salmon and trout spawning in the entire Lake Washington watershed. Redmond's original name was literally Salmonberg. Straightening the channel wiped out the wetlands, side channels, and shade that salmon need, and dropped the river's usable habitat by more than half.

Three species make this run in the river/slough: sockeye, Chinook, and coho. And the numbers tell the story better than I can. Counts exceeded 10,000 in the past and just over 100 in recent years. The straightened river that offers a tired salmon nowhere to hide.

King County and the Corps now spend real money on engineered log structures, bank restoration, and a floodplain project at Marymoor. They are essentially paying to reverse-engineer a fraction of what a "squiggly slough" did for free. As I've written before, this kind of federal flood-and-fish math is never simple: see my earlier posts on how difficult these tradeoffs actually are for government to sort out and on how federal flood programs keep landing on the wrong side of the Endangered Species Act.

So next time you're on the trail wondering whether to call it a river or a slough — just remember it used to be neither. It was a floodplain doing its job. We straightened it, named it twice, and are now paying to make it wiggle again, to allow the few returning salmon a chance to get home to spawn.