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Picture this: A big cottonwood on the edge of a river. The tree is full of nooks and crannies, providing shelter and nesting structure for a host of songbirds. Its leaves and bark attract insects and its hollow core provide a den for a black bear. Its roots, some of them exposed by the river, provide shelter from the current to westslope cutthroat trout.

But then the river rises and the tree topples. Silt builds up behind the trunk and the exposed root ball is a perch for an American dipper as it plunges into the icy water, literally flying underwater as it eats a cornucopia of insects living in the silt deposit and the nutrients the downed tree holds.

This is just one short story in the long life of a western gravel-bed river flood plain. Gravel-bed river floodplains are some of the most ecologically important habitats in North America, according to a new study by scientists from the U.S. and Canada, led by University of Montana professor Ric Hauer, director of the Center for Integrated Research on the Environment.

A good example of a healthy gravel bed river ecosystem is the North Foork of the Flathead River, Hauer said in a recent interview. The North Fork, protected on one side by the Flathead National Forest and the other by Glacier National Park, is largely intact. The key to keeping gravel bed rivers healthy is to allow the river to move back and forth across the floodplain. It’s the movement and shift of the gravel bed over time that creates biodiversity and cycling of nutrients.

Take the cottonwood, for example. It relies on flooding to spread its seed and to germinate new trees.

The river itself isn’t just water in a channel, either, Hauer and his colleagues note. It’s water flowing underground, through the gravel. The underground flow stays at a relatively constant temperature and comes to the surface often, depending on the typography. In some places, it creates surface seeps and sloughs, which are rich in nutrients.

The underground water is beneficial in two ways. It provides cold, oxygenated water for fish and aquatic insects in the hot summer months. And because its temperature is stable throughout the year, it provides relatively warm and open water in the winter months. The water is also rich in nutrients and promotes algal growth, which, in turns, draws aquatic insects, and the fish that feed on them.

This underwater flow, and the diverse ecosystem that surrounds it, extends more than a half-mile from the river channel itself, the study found.

Gravel bed rivers are less than 3 percent of the western landscape, Hauer notes. But they’re vital to dozens of species.

“This is where the magic happens,” Hauer said.

The rivers and their floodplains also draw a host of bird, mammal and amphibian species. Frogs and toads breed in the sloughs and ponds in the floodplain.

The highest bird densities and greatest bird diversities are associated with floodplains, the study found.

For larger mammals, like deer and elk, the gravel floodplains provide the earliest sources of browse in the spring. Wolves are not only attracted to the game, but they also den in the exposed gravel river banks.

A study in Banff National Park found that an estimated 40 percent of all winter wolf kills were in the river floodplain.

Intact floodplains also provide valuable grizzly bear habitat and travel corridors.

But the study also found that, paradoxically, the rivers can be barriers, where humans settle the floodplain. The study maintains that there needs to a paradigm shift in how floodplains are managed. Rivers, in short, need to roam. Humans often live too close to them and alter them with rip-rap and other means, which channelize the river.

Ironically, this degrades the very habitat that attracted people to them in the first place, Hauer notes.

Hauer notes that a 100-year flood plain doesn’t mean a flood will come every 100 years. The math doesn’t work that way. A big flood could happen one year and then happen again the next.

Building in a floodplain is just a bad idea. A flood the size of the 1964 flood will happen again, Hauer notes. We could even see a bigger one. In 1896, for example, a huge flood backed waters all the way up to what was then Demersville in Kalispell.

Hauer calls for greater conservation of rivers.

“We need to preserve those that are still functioning and restore those that are compromised,” he said.

The entire study can be read online in Science Advances at http://advances.sciencemag.org/content/2/6/e1600026.