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This dam protects jobs south of Seattle. Someday it won't

The Green River hasn’t flooded in more than half a century.

It used to all the time. Every other year or so, the valley filled with water and turned into one long lake, from Auburn, Kent, and Renton up to Seattle.

Now the area holds the largest collection of warehouse and manufacturing jobs in the state, worth billions of dollars. Someday, it will probably be under water again.

This realization hits hard for people nearby.

When Kent Mayor Suzette Cooke took office, she was prepared to deal with problems like traffic and jobs. But then in 2009, a storm surge dumped more water on the valley than the dam upriver had ever seen before.

After that, Cooke had to worry about floods. “The Green River would potentially have a biblical wall of water and put everyone under water,” Cooke said. “In places in Kent, it’s actually 20 feet deep, should it flood.”

It was like a return to the days before the Howard Hanson Dam. Back then, floods regularly buried the valley's farms, which is partly why the valley was so fertile.

But that era was buried in history, thanks to government officials who, over a hundred years ago, began work on a dream to tame the river.

The effort took off a couple of world wars later, once the nation better understood the importance of manufacturing. People knew taming the Green River would open land for factories and warehouses. In 1962, the Army Corps of Engineers built the Howard Hanson Dam at a pinch point in the river out past Black Diamond.

The dam has some concrete parts, but it’s mostly made of dirt and rocks — a big version of dams that kids build when they play in streams. It catches water running off the Cascades and spits it out below in a controlled stream. That stream is the Green River.

KUOW's Region of Boom team reports this month from Renton, Kent and Auburn. It's part of a project where we embed for about a month in communities outside Seattle. Where else should we go? Tweet @KUOW #RegionOfBoom

Daniel Johnson of the Army Corps runs the dam. He says the water constantly challenges engineers.

“What’s that hole, right there?” I asked, pointing to a large hollow in the hillside below the dam.

“It’s just a natural area, where it’s eroded out a cave, the water seeping down the hillside,” said Johnson.

Heavy equipment is working on that hillside now, improving the drainage so that holes like that don’t turn into bigger problems. If they didn’t tackle the small stuff like that, a hole in the wrong place could grow bigger as water rushed through, pulling parts of the dam with it.

That could cause the dam to collapse, like a child’s dam in a stream. But a half century of vigilance has paid off in the valley below. “The valley’s never seen a flood since the dam was built,” said Johnson.

Today, the Green River Valley south of Seattle has the biggest concentration of industry and warehouses in the state. Boeing is there, and Amazon’s Kent distribution center. Johnson said the dam is why there's so much industry in that region.

"Without that sense of security, without that ability to say, ‘We can manage the risk of these storm events,’ you couldn’t have done any of it,” said Johnson. “At most, it would have stayed farmland. And I don’t think you’d ever have a Boeing Field; you wouldn’t have had warehouses or server farms. That just wouldn’t have been practical.”

I wanted to see what’s grown up in the valley, to understand what could be threatened by a flood. So I went visit two places: a manufacturer and a warehouse.

I started at Omax, where a waterjet was cutting tiny metal butterflies out of a sheet of metal. It’s a demonstration project the company uses to show off its machines. Omax machines force a mixture of water and powdered garnet through a hole the thickness of a human hair. The water hits a sheet of metal with the speed of a bullet.

“It’s kind of like plugging your thumb over a garden hose," said Omax's Julene Bailie.

Bailie told me her company’s machines can cut through a bullet proof vest with water. She doesn’t find that strange. “The Grand Canyon was created by water, over time. But if you add pressure behind that water, and create a strong enough stream, you can cut through any kind of materials.” Companies use Omax’s machines to make everything from airplane parts to Christmas ornaments.

The company is part of the wave of manufacturers that spread across the valley floor after the dam went in, like mushrooms after a spring rain.

Here in the valley, you almost can’t hear the Green River when you’re standing next to it. The trucks make too much noise as they weave between manufacturers, warehouses and the port.

The Washington Shoe Company is in one of those warehouses. It’s a century-old company. Once they outfitted gold miners on their way to the Yukon. Today, they design quirky rain gear and have it made in China.

“We’re known for our character designs. Especially with children’s stuff. So here we have a raincoat. This particular one is Hello Kitty,” said Karl Moehring.

They also make those rubber boots with bug-eyed cartoon characters on them. If you have a little kid, you know them. “We make a frog boot, a ladybug boot, a kitty boot,” said Moehring.

Those boots arrive in the ports of Tacoma or Seattle from China. They’re split up into smaller boxes, and shoved down metal rollers.

Any given box might be heading to a mom and pop store, or to larger stores like Nordstrom or Target. It could be heading to the distribution facility that moved in just down the street.

Lots of companies are rushing into this valley. It’s building up pressure, like water behind the Howard Hanson Dam.

Which brings us back where we started, thinking about the dam’s limits. In 2009, the water rose higher behind the dam than the engineers have ever seen. Down in the valley, the levees almost flooded. Sandbags were deployed, and some of weak spots in the dam were strengthened. Since the problems emerged in 2009, the dam has prevented an estimated $6 billion in flood damages to the valley below.

Daniel Johnson, with the Army Corps, says his organization has learned humility. “There was a time in our nation when the ‘we can conquer anything mentality’ was very strong. ‘No river is too big, no mountain is too high, no ocean is too wide.’ There’s nothing we couldn’t do. But at the same time, you have to realize, not everything can be conquered entirely,” he said.

Standing on the Howard Hanson Dam, Johnson is proud of what his engineers have done. But he knows they’re not gods.

“We’ve done a lot on this site to build a very good dam. And it does a lot of great things for people downstream,” he said. “But at the same time, it can’t do everything. And there’s going to be a storm someday that’s bigger than this dam can handle.”

When that happens, the dam may have to release more water into the Green River than the levees can hold to protect the dam from collapse. When that water pours over the levees, the valley could face a catastrophic flood.

Hopefully, we’ll all have plenty of warning before the water rushes in, and the only objects floating in the valley will be boxes of rubber boots.

Joshua McNichols can be reached at Have a story idea? Use our story pitch form.

Correction: The water in the Green River originates in the Cascades, not on Mt. Rainier, as this story originally indicated. The Green River did have Mt. Rainier water in it once, but after a 1906 landslide and the water wars of the early 1900s, that changed.

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