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Like a sponge

Research led by UM underscores importance of gravel-bed rivers to safeguard biodiversity

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When Norman Maclean wrote “eventually, all things merge into one,” in his novella “A River Runs Through It,” river ecologist Ric Hauer didn’t expect to take the idea so literally.

But in 2013, Hauer was giving a lecture on gravel-bed rivers at a Yellowstone To Yukon conference, and he saw one of the region’s foremost grizzly bear biologists slap his head in amazement.

“I’m going to have to change my entire research program based on what you just told me,” Hauer recalled Mike Proctor saying. “Because you’re explaining, from a standpoint of a major feature of the landscape, why my bears are doing what the bears are doing.”

A year later, University of Montana elk-and-wolf expert Mark Hebblewhite had an identical experience after hearing the same talk. He told Hauer he needed to completely rethink what his elk and wolves were doing. So did Clint Muhlfeld, who studies fish habitat at the Flathead Lake Biological Station.

Hauer himself has been studying river ecology for the past 40 years. Now the director of UM’s Center for Integrated Research on the Environment, he’s considered one of the leading experts on how rivers in the northern Rocky Mountains function.

But as he spent more time with a growing network of regional scientists, he became increasingly convinced that new discoveries are best sought where his field touches the boundaries of other disciplines.


What Hauer said – what produced these eureka moments around the roundtable – goes like this:

People tend to think of rivers as water running down a rain gutter. Sometimes it dries up. Sometimes it overflows. But it’s just a ribbon running between two walls.

“That perception is completely wrong,” said Harvey Locke, co-founder of the Yellowstone To Yukon Conservation Initiative and co-author of the scientific paper where this all comes together.

“It’s more like a wet sponge," Locke said. "Imagine that whole system from valley wall to valley wall being a big gravel sponge. Sometimes that sponge is half-full of water. Sometimes it’s totally soaked. When it’s totally soaked, that’s a flood. When it’s drying, you won’t see any water in the channel at all. But it’s not about the channel. It’s that whole wide system.”

The influence of gravel-bed rivers like the Clark Fork or the Bitterroot extend far beyond their banks. Hauer’s research showed the water seeps through the cobbles and boulders underground for hundreds, even a thousand meters beyond the riverbank.

In spring runoff, the water spreads out and nourishes plant and microbe communities throughout its floodplain. Water flowing back into the main channel leaves behind impurities as it filters its way through the gravel.

That underground flow stays insulated from the surface weather, staying cool in summer and unfrozen in winter. Interfere with that process, and a delicate web of life unravels.


Missoula offers an easy-to-see example. Compare the new growth carpeting the rebuilt floodplain above the Clark Fork River’s former Milltown Dam to the decrepit and collapsing cottonwood stands in the city center, where the riverbank has been fortified for most of a century. Without access to the underground floodplain water exchange, those shoreline forests can’t support saplings and regeneration.

Flatland rivers running on compacted silt and sand don't show this effect. They still flood over their banks, but only penetrate the fine sediment a meter or two a day. Gravel-bed rivers spread water underground hundreds of meters a day.

The difference can be seen on the Flathead River up- and downstream of Kalispell. Above town, biodiversity in the gravel-bed channel is significantly greater than between Kalispell and Flathead Lake, where the river runs over an ancient delta of packed silt. There, the riparian ecosystem barely gets beyond the riverbanks.

Why does this matter? Because in the Rocky Mountains, gravel-bed rivers make up about 3 percent of the landscape. But they’re home to 60 percent of the plants and animals that live here. Of the 235 bird species native to the northern Rockies, 200 of them spend a significant portion of their lives on riverbeds. About half of those nest there.

So when Muhlfeld, the fish expert, looked at where bull and cutthroat trout were spawning eggs, it was always on gravel-bed streams where underground seeps brought fresh oxygen to the egg redds and flushed away the waste that might suffocate the embryos.

When Proctor checked his grizzly satellite collars in spring, the locations were always in floodplains, where bears were grazing on the first plant shoots.

When Hebblewhite plotted his wolf den locations, they all fell within short distances of river bottoms. So did most of the kill sites where they took down elk. Three weeks ago on a trip to Yellowstone National Park’s Lamar River Valley, he and Locke watched a wolf take an antelope fawn to a den near Slough Creek, just off the gravel-bed floodplain.

“There are other flat places they could be, but they’re not,” Hauer said. “They’re on the floodplains. The floodplain is the arena. It’s not just a corridor where animals go up and down.”


Hauer and Locke had worked together for several years building support for the Transboundary Flathead River that flows out of British Columbia into Montana along the western edge of Glacier National Park. That campaign eventually produced a memorandum of understanding between the province and state that was later ratified by both the Canadian and U.S. governments, protecting the Flathead from future mining and energy development.

As they built their arguments justifying the Flathead’s significance, they realized how much the flowing water sloshed into other scientific disciplines beyond their own. So in February 2015, they got together with Proctor of Birchdale Ecological in British Columbia, riparian botanist Stewart Rood of Alberta's University of Lethbridge, and UM colleagues Hebblewhite, Muhlfeld, conservation biologist Victoria Dreitz, amphibian expert Winsor Lowe and restoration ecologist Cara R. Nelson. The group outlined their collective vision of a research paper and got to work. They had a final version drafted by November.

“You cannot submit to a higher profile journal than the Science AAAS journals,” Hauer said, refering to the American Association for the Advancement of Science. “A typical paper in Science Advances magazine is three pages - max, maybe four if there’s lots of graphics. This is a 13-page paper. That is monster.”

An emailed statement from Science Advances’ editorial staff said the paper highlights future directions of interests for not only the scientific community, but policymakers and the public.

“In their review, Hauer and colleagues evaluated decades of research pointing to gravel-bed rivers and their floodplains as hotspots of biodiversity,” the editors wrote. “Their final analysis implies a need to modify development so that it is less impactful on these important ecosystems.

"It does not proscribe development per se, but rather, should lead to development approaches and designs that are creative and adaptive to environmental efforts that seek to protect biodiversity.”

“This kind of information is really helpful understanding how river systems are connected to upland systems,” said Yvette Converse, coordinator of the Great Northern Landscape Conservation Cooperative, which brings together government and private land managers. “This paper shows how they relate to the timber industry, the mining industry, to roads and transportation. It helps us understand the changes we see developing dams and water storage. Rivers aren’t just the water that comes down them.”

Most of the authors of the paper limited their comments to the data and observations developed in their research. But Locke allowed himself to go a few steps further on those directions of interest.

“This is like having a new pair of glasses for seeing how rivers work,” Locke said. “And they show our relationship with the natural world is wrong and we need to fix it.”

Those relationship problems extend from local zoning rules about building houses in floodplains to plans in India, Nepal and China over hydroelectric dams on Himalayan rivers. They include new awareness of what threatened species like bull trout and grizzly bears need to survive from gravel-bed rivers. They affect how the Confederated Salish and Kootenai Tribes manage flows out of Kerr Dam to restore the lower Flathead River, and what might happen if Congress approves requests to renovate Gibson Dam on the Rocky Mountain Front near Choteau.

“When we stabilize a gravel-bed river that should be mobile and dynamic, we’re locking it into place,” Hauer said. “If we lose the functionality of a gravel-bed river, we drastically lose the natural heritage we have in western Montana.”

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