NYT: Like Wildfires, Giant ‘Reset’ Floods Not Only Destroy—They Renew

The floodwaters that roared off the Yellowstone plateau and ripped through southern Montana in June have altered so much of the Yellowstone River that whitewater raft guides said they would have to relearn how to float the changed route. [bold, links added]

The flood, fueled by torrential rains that fell atop melting snow tore out much of the river’s template — the physical features of the river — and built a new one.

Its swell was measured at over 51,000 cubic feet per second just north of the park, far surpassing the high of more than 32,000 in 1990s flooding.

It’s the type of event that occurs here once every 500 years; floods of this magnitude are known as reset floods. …

In the midst of the damage and dislocation caused by overflowing rivers, experts point out that floods play a vital ecological role over time.

They are messy and chaotic — the technical term is disturbance regimes. Floods are considered a shot of adrenaline for the evolutionary survival of river systems.

Like conditions wrought by wildfires, once the flames and smoke die out, nature begins to rebuild.

“A common flood that goes over [its] bank re-waters the landscape, rejuvenates the vegetation, and leaves a veneer of fresh sediment,” said Jack C. Schmidt, a professor of watershed science at Utah State University and the director of the Center for Colorado River Studies. “That’s a fertile situation that allows a good seeding ground for cottonwoods.”

A flood of the magnitude that hit Yellowstone does even more.

“A gangbuster reset flood is a whole different deal,” Dr. Schmidt said. “It rips up the oldest of trees. It sends the river off into a new direction. It re-channelizes the flood plain and rejuvenates everything and gives the river a new lease on life.”

While floods do cause destruction of the natural world, Dr. Schmidt said that post-flood benefits greatly outweigh damage as the years pass.

A river that floods seems almost alive, constantly remaking and renewing itself. It’s not just the physical features of the river that are reborn; the living things that make their home in and along the river and have adapted to periodic flooding, are also renewed.

This natural ecological rebuilding process, however, is endangered.

Experts are concerned about the effects of the surge in dam building on river systems, stemming from economic benefits. … Only one-third of the world’s rivers remain free-flowing. (The Yellowstone is the longest undammed river in the United States at nearly 700 miles.)

Dams bring a great many benefits, from power generation to flood prevention, but they also have considerable effects, especially those caused by the end of high water levels downstream.

The periodic flooding and receding of a river are called a flood pulse, and while it can cause problems for people who live along a stream, it also turns rivers into biodiversity hotspots.

The floodplain and the river channel are a single system.

A free-flowing river, over time, behaves like a fire hose, snaking across a landscape much broader than the main channel when it floods, creating and nourishing a rich patchwork of marshes, swamps, oxbow lakes, riverside channels, and ponds while depositing sediment and other debris throughout its system.

“That rhythm of flooding is part of the heartbeat of rivers,” said David A. Lytle, a professor of integrative biology at Oregon State University. “One of the important things about flooding is the reconnection of off-channel habitat back to the main stem of the ecosystem,” which allows fish, amphibians, and other species to return to the river and nutrients to be exchanged, which fertilizes the food chain.

“These systems are dynamic,” said Paul Keddy, a former professor at Louisiana State University and the author of “Wetland Ecology: Principles and Conservation.”

“When people build dams to control the spring flood pulse, that has immediate deleterious effects,” Mr. Keddy said. “As soon as you cut out the spring floods, wetlands down the whole watershed start to shrink back toward the river.” …

A flood in 1941 sent a huge amount of sediment down the Rio Grande and created a fertile bed for the beginnings of the bosque. But the flood also wiped out farms and towns.

In the 1960s, construction of the giant Cochiti Dam, 50 miles north of Albuquerque, got underway to thwart the flow of water and sediment down the river. It worked — at a cost.

The dam also ended the flood pulse, which has prevented young cottonwoods from establishing, leaving only the eight-decade-old trees that grew up after the flood. Craig Allen, a retired U.S.G.S. ecologist in Santa Fe, N.M., calls it a “zombie forest.”

“It’s the living dead,” he said. “The whole riparian system has been transformed into something much drier.”

Invasive fire-prone species of tree, such as tamarisk, have moved in beneath the old cottonwoods. Bosque forest fires, once unheard of, are common.

Dams also cut off the gravel, silt, and other sediments that rivers carry, which are used to build new ecological features during a flood.

Fine sediment trapped behind the dam contains essential nutrients “and the base of the food chain is undermined,” said Matt Kondolf, a professor of environmental planning at the University of California, Berkeley.

Because the dam also reduces stream flow, “it simplifies the channel,” he said. “So, where before you had gravel bars and pools and riffles, all of that gets washed away, and you wind up with a bowling alley geometry. If there’s a fish in there, there’s no place for them to hide, they just get washed downstream.”

Floods also stimulate life in the main river channel by capturing and bringing nutrients from the flood plain into the main channel, which provides more food for insects, which in turn feed other creatures.

After a major flood at a study site on Sycamore Creek in Arizona, Dr. Lytle said the waterway, with downed trees and heaps of mud, looked like a disaster.

“Debris was piled everywhere,” he said. When he and his team began looking below the stream bed, however, they found more mayflies than they had ever seen.

“A big pulse of nutrition was triggered by that major flood,” he said. “The conditions were just right for them to explode in numbers. That means more food for birds, lizards, spiders, and fish.”

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