From Boise

Valley of the dammed

Published 10 days ago • 15 min read

I'm sure you've seen the river at some point in the last few months - it's rippin. Technically, the Boise River floods every year, yet those "floods" are not anywhere near as intense as in years past. But some of those past floods were incredibly damaging. Like on July 4, 1862, when a flood reportedly reached from Boise all the way to Caldwell. Another flood in 1959 had bulldozers pushing mud out of Boise streets and residents shoveling 10-inch-deep mud off their sidewalks and driveways. Crazy, right?!

You'll learn all about the history of flooding on the Boise River – and how it was stopped – in today's story, which was written by Sharon Fisher. Listen to it on our podcast!

Valley of the dammed

By Sharon Fisher

This is the time of year when people worry about flooding along the Boise River. While it’s much less of a concern nowadays, it wasn’t that long ago that Boise River floods were a major issue. And it’s not out of the question that floods could continue to be a problem into the future, especially with climate change.

How the Boise River was formed

Somewhere between 2 and 9 million years ago, the western Snake River plain in which the Boise River flows today contained a giant lake called Lake Idaho. Then, sometime between 2 and 4 million years ago, Lake Idaho drained, and in the process created what we now know as the Boise River, according to the Geomorphic Assessment of the Lower Boise River, Idaho.

More recently, there was Lake Bonneville. Around 18,000 years ago, a prehistoric lake the size of Lake Michigan broke through a natural dam in southern Bannock County, pouring half of its volume into the Snake River, dumping boulders and sediment, creating Shoshone Falls, and carving canyons all the way to Lewiston and the Columbia River. But the Boise River back then wasn’t like the Boise River we know today. In fact, prehistoric geological evidence indicates that the river changed its path a number of times, each time leaving former floodplains known as terraces (some of which are what we know as the Boise Bench). The lower Boise River has at least eight terraces, showing the previous flows. Native American oral histories also describe a number of floods in the Pacific Northwest, some of which could have been the Boise River.

But even historically, though, the course of the Boise River has changed. “Historic accounts and maps from the 1800s suggest the Boise River was primarily single-threaded with several islands in the area near the cities of Boise and Eagle,” the Geomorphic Assessment noted. “The channel in this area was described in 1834 by John Kirk Townsend as ‘a beautiful stream, about one hundred yards in width, clear as crystal, and, in some parts, probably twenty feet deep.’

Early drawings from farther downstream suggest the lower 30-40 miles of the channel became primarily multi-threaded with ‘many islands’ as depicted on David Thompson’s 1818 Map of the River. To geologists, there’s plenty of evidence along the Boise River of its history of flooding. “Rivers move by their nature,” said Claudio Berti, Idaho’s state geologist and head of the Idaho Geological Survey in Moscow. “They change their course through their floodplain, and build surfaces and deposits. The river goes through phases of accretion and incision, and there are deposits and rocks that testify for these long histories of evolution.”

For example, floods also helped provide the fertile soil that made the Treasure Valley a great place to settle. “A lot of the soil we have is from floods,” Berti said. “Floods have a regenerating effect on soil coverage, which is very important because of the redistributed sediments and nutrients.”

Without flooding, the Boise area wouldn’t have these natural features to which we are all accustomed.

What makes a flood?

Floods tend to be a problem primarily because people build close to the water. Rivers provide not only water, but also transportation and energy through hydro power. Consequently, cities tend to be built near rivers, but that exposes the cities to the risk of flooding. In the case of Boise, two factors contribute to its risk of flooding: the Boise River collects snowmelt through the spring and tributaries of the Boise River contribute to its flooding potential.

“Some of the more well known historical flooding is related to tributaries,” such as the 1959 flood that was caused by a mudslide through the tributary the Cottonwood Creek, which affected the Warm Springs area, Berti said. As the area began to be settled by white people, floods began to be seen as something to be prevented.

Boise’s early flood history

Ada County Emergency Services has published a long list of Boise River floods, and back in the 19th century flooding happened every year or two.

One of the biggest reported floods was also one of the earliest, on July 4, 1862. “Boise historian Merle Wells told the story of homesteader Isaac Coston, who reported that on July 4, 1862, all the land in the river bottoms ‘extending from bluff to bluff and from the present site of Boise westward to the canyon near the present site of Caldwell’ was under water,” reported Rocky Barker in the Idaho Statesman. “Engineers estimate that the river that summer ran at 100,000 cfs, 16 times the flow today.”

On September 1, 1864, floodwaters in downtown Boise reached 4’ deep due to a flash flood on Cottonwood Creek. In fact, Cottonwood Creek continued to flood almost every year thereafter. The next big flood on the Boise River itself (though it flooded Cottonwood as well) was in 1871. It flooded low-lying areas from the Bench to the Highlands and was estimated to have resulted in 5,500,000 acre feet of water. An 1876 flood destroyed the Ninth Street Bridge downtown. In 1881, a flume was built near Cottonwood Creek for irrigation, and floods in 1892 and 1894 damaged it.

But that was nothing compared to the flood of 1896, which is considered to be one of the largest in the Boise River in historical times. It lasted for more than a month, with an estimated flow of 35,500 cfs. The second largest was just a year later, in April 1897, with estimated flows of 29,500 cfs.

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What is a dam?

It was in response to these floods – as well as to the desire from farmers to have irrigation water available year-round, and the development of hydropower – that led to the development of three major dams in the Boise area. Arrowrock Dam, Lucky Peak Dam, and Anderson Ranch Dam.

Dams aren’t always near the areas they’re protecting from floods. Siting a dam is related to three factors, Berti said. First, there needs to be a natural restriction in the valley to make it easier to build a dam, such as an area where a river goes through a gorge. Second, especially if it’s going to be used for irrigation, there needs to be enough room in the back, lined with bedrock for a basin to store water. Finally, if it’s going to be used for irrigation, there needs to be a way to attach its power to a grid.

Dams are also controversial, particularly more recently as the ecological damage they produce has become more obvious. As Berti said, “They have a major impact on the natural course of a stream.” They also have the paradoxical effect of flooding a large section of territory to prevent flooding elsewhere, by creating a reservoir behind the dam. Another paradoxical effect is that dams – especially ones with multiple purposes such as both irrigation and flood control – can end up creating floods on their own., Berti said. “There’s pretty good science that shows that the mismanagement of a dam can actually be more catastrophic in flood control,” he said.

The Army Corps of Engineers and the Bureau of Reclamation, which manage the three dams near Boise, do an intricate dance to balance the two needs, said Rob Tiedemann, President of The Boise River Enhancement Network (BREN). “Irrigators would like the pools behind the dam to be full to the brim,” so they have water all summer, he said. “Those downstream communities would like to see the pools empty so they can accommodate flood flows and prevent floods from occurring downstream.” The two agencies work hard to satisfy both entities, he said.

The building of Arrowrock Dam

In 1902, the federal Bureau of Reclamation was formed with the goal of controlling water and “reclaiming” land for agriculture and settlers. Idaho was high on the list, and Arrowrock Dam was soon planned as the Bureau's second project. It was built from 1911-1915, and located, ironically, near the site of the house of Mary Hallock Foote and her husband Arthur De Wint Foote, who helped design Boise’s irrigation system. It holds 272,200* acre feet of water.

Because of the floods themselves, construction of the dam proved to be quite the engineering feat. Engineers even considered using the same sort of caissons that were pioneered during the construction of the Brooklyn Bridge, but instead routed the water around the project.

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Completed in 1915, Arrowrock was considered to be the highest dam in the world until 1932.

Unsurprisingly, the floods kept on. On July 24, 1913, 25 blocks of downtown Boise was flooded out, and in April 25, 1936, rain and melting snow led to an estimated 19,700 cfs flow, washing out bridges and roads and flooding farmland.

*Some sources differ on the capacity of the reservoirs.

The building of Lucky Peak Dam and Anderson Ranch

Work on Anderson Ranch Dam, which like Arrowrock was primarily intended to retain water for irrigation, started in 1941, but was delayed due to World War II. (In fact, Japanese-American internees at the so-called Minidoka Relocation Center were enlisted into the work – which required an act of Congress, because the Bureau of Reclamation operated under a 1902 law stipulating that “no Mongolian labor shall be employed.”) It was located about 42 miles upstream of Arrowrock and was intended to hold 418,000* acre feet of water.

But during the delay, Boise suffered another devastating flood in 1943, when damage from a 25,040 cfs flow reached almost $1 million, 11 bridges were closed for at least five days, 200 families were evacuated, and numerous businesses were flooded.

Consequently, “in 1946, Lucky Peak Dam was authorized under the Flood Control Act. Lucky Peak was named after a successful gold mining camp about three miles north of the planned project footprint, just downstream of Arrowrock,” writes Hannah Mitchell for the Walla Walla District of the Army Corps of Engineers. Unlike the other two dams, which were built by the Bureau of Reclamation, Lucky Peak was built by the Army Corps of Engineers, and intended primarily for flood control, holding 264,400* acre feet of water. Construction began in 1950, the same year Anderson Ranch Dam was completed, and was completed in 1955.

Early visitors to Lucky Peak thrilled to see the rooster tail. “The Rooster Tail uses two of the dam’s original six gates to release water from the reservoir back to the Boise River,” Mitchell writes. “Using a ‘flip bucket,’ the spray is aimed high into the air to reduce the water’s scouring, erosive force before it falls back into the stream channel. When Lucky Peak Dam was built, Rooster Tail displays occurred regularly because the outlet gates made up the only release structure of the dam.

But to engineers, the sight represented wasted energy. Consequently, power generation was added to Lucky Peak’s features, starting in 1988. Power generation was added to Anderson Ranch Dam in 1951, and to Arrowrock in 2011.

Post-dam floods

Even Lucky Peak didn’t end the floods completely. “In 1959, a fall heavy rainstorm dumped 2.23 inches of water on top of recently burned lands on Shaw Mountain, causing mudflows to wash into the city of Boise from Cottonwood Creek, Maynard Gulch, and other streams that flowed out of the eastern foothills,” wrote Steve Stuebner for the Boise River Flood Control District #10.

A video made at the time shows bulldozers pushing mud out of Boise streets and residents shoveling 10-inch-deep mud off their sidewalks and driveways.

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But while the dams didn’t stop flooding completely, they vastly reduced it. “Upstream flood regulation, as afforded by Lucky Peak, Arrowrock, and Anderson Ranch Reservoirs, has substantially lowered flood flows experienced on the Boise River,” noted the 1976 draft environmental impact statement for Lucky Peak modification. “Regulation of the December 1964 runoff which, without regulation, would have presented the greatest known outflow from the system, resulted in a flow reduction from 44,000 cubic feet per second to 300 cubic feet per second at Boise. This amounted to a crest reduction of 14 feet.”

Similarly, a 1983 flood was measured at 24,290 cfs as it started, but the dams reduced it to 9,840 cfs, Stuebner writes. It was caused by what Tiedemann called a “rain on snow event.” A February rainstorm caused the million acre feet of snowpack in the mountains to melt and move into the reservoirs more quickly than usual. “If it rises too quickly and dams are holding all the water, we have a phenomenon where the river behaves as it did in the past without dams, and conveys the water delivered to it.”

Water was lapping to the top of the spillway at Lucky Peak, which had never been used. “We were less than 48 hours away from having the spillway activated, with the unknown of whether it would function,” Tiedemann said. Fortunately, the weather changed dramatically – it got colder, the rain turned to snow, and the threat dissipated. Nonetheless, “they had to manage flows very carefully that season,” he said. But while it still flooded thousands of acres and damaged property, it wasn’t nearly as devastating as the larger flow would have been.

*Some sources differ on the capacity of the reservoirs.

The dam that never was

Another dam that was considered, but never built, would have been at Twin Springs, about 20 miles north and east of Arrowrock. While the site had reportedly been considered for a dam by USBR since the 1910s, (and again in 1938, Stevens wrote) it was proposed as a specific project around 1966, with construction planned to start in 1971.

What was notable about the project was not the dam itself, but how it was planned to be built: As part of the U.S. Army’s Project Travois, which was intended to demonstrate how nuclear devices could be used in public works construction projects, the dam was proposed to be constructed through nuclear explosions.

“It is expected that the detonation of a 40-kt explosive [Hiroshima was 15 kt] placed 685 feet from the nearest free surface in terrain with a 30-(degree) slope would produce in excess of 7,000,000 yd 3 of quarry rock,” noted a report from the 1970 Las Vegas conference, Symposium on Engineering with Nuclear Explosives. “This is more rock than is needed to construct the Twin Springs dam.” The dam would either have been constructed from the rubble, or the explosion would create a landslide directly into the river.

Construction using nuclear excavation was expected to save $1 million, though “This savings does not include the expanded operational, safety, and technical programs that would be conducted during the experimental stage of any proposed application,” the report noted.

A public hearing was held on the project in Boise in August 1968 with some preliminary nuclear explosion testing scheduled for spring 1969, but the project was cancelled altogether about ten weeks later with no reason given, according to the 2011 report.

Flood risk today Despite the three dams, the Boise River still has a flood risk. Flood stage is defined as 7,000 cubic feet per second (cfs), but flooding can actually start as low as 4,000 cfs, according to the National Weather Service.

In fact, technically the Boise River floods just about every year, but it’s not necessarily damaging or dramatic. “Good engineering offers areas of natural expansion,” Berti said.

In the case of the Boise River, that means the Greenbelt, as well as Boise’s “Ribbon of Jewels,” or its riverside parks. Along with providing beauty and recreational opportunities, the parks and Greenbelt keep development away from the riparian areas that are more likely to flood, and the parks themselves can absorb natural, low-magnitude flooding, Berti said.

Other organizations, such as the Boise River Enhancement Network (BREN), advocate for keeping open space next to the Boise River and work on projects that help protect the riparian area, such as planting trees and repairing river banks – in fact, repairing damage caused by flooding, said Rob Tiedemann, president of BREN and owner of the Ecological Design Inc. consulting company.

Previously, banks were repaired with “rock riprap,” or angular rock sized to withstand flood flow. “You stack it, so you build a wall with the hope that it protects the bank,” Tiedemann said. But because the rock is hardscape, it doesn’t provide a surface to grow vegetation, which is not only aesthetically unappealing but provides a habitat for fish and wildlife.

(It could be worse. Before rock riprap, river banks were sometimes stabilized with old auto bodies, some of which are still there. “That was the best solution they had at the time,” said Mark Zirschky, manager of the Boise River Flood Control District #10, in an email message.)

It’s also not uncommon for rock riprap to fail. While it relocates the river, it typically still has the energy to be able to scour banks and cause damage, Tiedemann said.

That’s what happened when an area of the Boise River near Kristin Armstrong Municipal Park flooded in 2017, after the so-called “ Snowmageddon .” The river rose above the level of the rock riprap and the bank above it collapsed. “What we have now is rock riprap to the water line, then we have a bench that’s rather narrow, then a standing bank that’s vertical that takes us to the ground surface,” Tiedemann said. It’s a hazard, not just to the people near the bank, but the people who are trying to get out of the river at that spot, he said.

BREN has now received a grant to repair that damage and hopes to start construction in the winter of 2024-2025. “Instead of rock riprap, we’re going to do a strategic retreat and pull material away from the river, creating a larger channel, reducing the risk of floods, and revegetating the banks with plants common to the floodplain,” Tiedemann said.

The Boise River Flood Control District #10, founded in 1971, performs flood risk management, flood response, and flood recovery, Zirschky said. Flood risk management includes items such as removing debris from the river channel, removing trees that are in danger of falling into the river channel, and assisting landowners with bank repairs, such as after 2017, he said.

The organization has also developed the Boise River 2-Dimensional Model Tool, which the district will use to develop a channel maintenance plan to monitor the river and plan projects, such as tracking sediment deposits at the head of Eagle Island, Zirschky said.

Boise’s Public Works Department also operates and maintains a series of flood control structures on Stuart Gulch, Crane Gulch, Hulls Gulch and Cottonwood Creek in the Boise Foothills, according to the department’s website, but cautions that “these will not completely protect property in an extreme flooding event.”

Flood risk in the future

What’s harder to predict is how likely the Boise River is to flood in the future, particularly with the effects of climate change. While the likelihood that the Boise River will increase its overall discharge is not very high, what could change is the pattern at which the flood season will have its peak, Berti said. “The season is definitely changing,” he said. “We see that already.”

The result could be that the highest flows will shift to earlier in the season, and organizations that use the water for hydro power are already starting to gear up for that, Berti said. “Today we’re at a sweet spot with the precipitation patterns and melting,” he said. “The water makes it to the river system when there’s the most demand for hydro power. That’s probably not going to be the case in the near future. The maximum flow will come too early for summer cooling.”

That will lead to more pressure to keep water upstream in the river system and in reservoirs for longer, Berti said. In fact, more storage basins may be needed to maintain the ability to do hydro power, he said. There’s also concern about Arrowrock’s age, and whether it would be possible to replace or raise it in today’s environmental climate.

And the Boise River is still subject to what’s referred to as a “100-year flood, Tiedemann said. That’s 16,500 cfs, and there’s a 1% chance of that happening every year.

Want to see what a 23,900 cfs flow would look like? Check out this simulation.

Thanks for reading!

With love from Boise,



From Boise

by Marissa Lovell

A weekly newsletter & podcast about what's going on in Boise, Idaho. Every week we share stories about people, places, history, and happenings in Boise.

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