A Silver Lining: Giant Floods Not Only Destroy, They Renew

GARDINER, Mont. — 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.

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 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.

Experts say the flood, as well as recent record flooding in Australia and southern China, is likely driven by a warmer atmosphere, which can hold more moisture; and that more floods like this are forecast as the world heats further.

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 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 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. Dam building has become especially pronounced as the climate changes. 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 is called a flood pulse, and while it can cause problems for people who live along a stream, it also, turns rivers into biodiversity hot spots. 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, river side channels, 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.”

An absence of flooding can wipe out species that have adapted to its cadence. Some species of fish, like the silvery minnow in the Rio Grande, are endangered, in part because of the lack of a flood pulse that triggers spawning. Riverside cottonwood galleries, known across the southwest as bosques, are imperiled. Bosques are the tall, majestic, deciduous forests common along rivers, shady oases in the desert that provide habitat for a broad range of bird, mammal and reptile species.

The bosque along the middle Rio Grande in New Mexico is the largest such cottonwood forest in the country, stretching nearly 200 miles across New Mexico.

Cottonwood seeds are borne on white cotton-like puffs — hence the name — that sail through the air.

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 sediment 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.”

Removing floods from the landscape reduces the number of ecological niches and species, simplifying the ecosystem. Wet places along the river dry up — a trend that climate change accelerates — and become less resilient.

Dams not only end the flood pulse, they also change water temperatures. The river above the dam becomes a lake where many native fish species cannot survive and warm water fish, such as small mouth bass, thrive. Below the dam, the natural flow regime that many species are adapted to is altered.

One ecological change dams bring to rivers is caused by something called hydropeaking. Dams that generate power cause daily fluctuations in water levels as power is needed. Aquatic insects, such as caddisflies and mayflies, though, are adapted to natural seasonal flooding and lay eggs just below the surface of the water. These frequent drops in water levels can dry out and destroy the eggs and eliminate species of insect and make the ecosystem much less diverse.

Floods are so critical for the health of rivers that biologists and managers of many dammed rivers are managing dams to restore some semblance of flooding, something called adaptive management. This is critical, experts say, as the climate warms.

Some dams, for example, are managed to allow sediment to be flushed into the river below. Dr. Schmidt, for example, has researched the effects of releases from Glen Canyon Dam on the Colorado River to restore gravel and sandbars.

Climate change, however, may make adaptive management harder. This year, the artificial floods that Dr. Schmidt has engineered were curtailed because of extremely low water levels in the Colorado River.

“It’s a higher order of challenge to maintain resilience in systems that are going to get hit really hard, more often, by multiple kinds of extreme events,” said N. Leroy Poff, a professor of biology at Colorado State University. “We need a more serious national effort to identify systems that are most vulnerable to extremes.”