How Boise's 1959 Mudslide Led To Lasting Protections For City's Foothills

Apr 18, 2014

The Idaho Daily Statesman, August 21, 1959.
Credit The Idaho Statesman / The Idaho Statesman/Boise Public Library

It was August, 1959. Boise was having one of its typical hot, dry summers. A fire had just burned 9,000 acres in the nearby foothills. Then on August 20, a huge storm system dumped heavy rain on the Treasure Valley. One inch of rain fell in an hour on the burn scar. 

The water overwhelmed the hills and washed away tons of topsoil. A Forest Service video made several years after the event, tells the story.

“The flood brought destruction to Boise’s eastern residential section and business district,” the video’s narrator explains. “Property damage ran high.” 

East Boise was covered in a delta of mud. No one was killed, but some people asleep in basement bedrooms had to escape flowing mud that ended up being 10 inches deep in some spots. The government’s video shows residents standing in their yards shoveling mud, and bulldozers pushing tons of topsoil that used to cover the nearby hills. Boise was a mess.

Preventing Future Disasters

While homeowners cleaned up, leaders of several federal agencies were worried. If such a thing could happen once, it could happen again. Pressure to find a solution increased when two more smaller floods and slides occurred over the next few weeks. 

Soon, officials with the Boise National Forest, the Soil Conservation Service and the Bureau of Land Management announced what they called “The Boise Front Watershed Restoration Project." They would dig trenches along the contours of the hills to catch heavy rains and prevent future mudslides. Work began that October and lasted several years. The project created the series of grooves in Boise’s foothills that that can still be seen today.

“In a lot of ways it was a major wake up call," says Boise historian Jennifer Stevens. She’s studied and written extensively about Boise’s foothills. Up until the 1950s, Stevens says, the foothills really weren’t thought of as much an asset. They’d mostly been used to graze sheep. But that had left the hills void of much of their native vegetation. Homes had also popped up in the area, sometimes in locations that weren’t good for the land.

Stevens says it was clear the effects of the fire and the storm were made worse by human practices. Soon, a handful of people who’d never rallied around an environmental cause, were doing just that.

“It mobilized them to start putting pressure on our elected officials to create polices that would put some regulations on the kind of building we were doing in those areas,” Stevens says.

Years Later, Science Results In Action

The government championed its collaborative effort to put grooves in the foothills as essentially fixing the problem. Stevens says the pace of haphazardly building in the foothills actually increased in the early 1960s. 

Fast forward to the spring of 1973, when a Boise State geologist by the name of Kenneth Hollenbaugh wrote there was an active landslide area in the foothills east Boise. Stevens says that report galvanized the movement to protect the foothills because it brought hard science to the discussion.

“You had this event in 1959, but you could kind of explain the event away,” she says, referring to the fire and rare deluge. “But as soon as you get science in there saying ‘you know this is active landslide and we can’t build on every square inch in the foothills'.”

Even Idaho's political elite couldn't escape 1959's deluge of flood water and mud. Here, Gov. Robert Smylie digs out.
Credit The Idaho Statesman / The Idaho Statesman/Boise Public Library

A Disaster’s Legacy

Stevens thinks rules that limit foothills development in Ada County passed as late as the 1990s, can even be traced back to a movement that began when Boise cleaned up nearly a foot of mud and debris more than three decades earlier. As the region’s population has swelled with newcomers in recent decades, she thinks the mudslide’s role in shaping how Boise’s foothills look today, is under appreciated.  

“I don’t think a lot of people know about the ‘59 mudslide,” Steven says. “And I don’t think that a lot of the people who live in the foothills have a great awareness [of] foothills planning and sort of the history behind that. I think at the same time, part of the reason people move here is because our foothills aren’t completely covered with houses. There’s still a lot of open space up there.”

Here's the U.S. Forest Service's video explaining the 1959 flood.

Don't miss: Adam Cotterell reports on the science behind the slide and the chances a similar event could happen today.  

The Boise Pubic Library, the City of Boise and Boise State University's Albertsons Library contributed to this report.

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