A Silver Liner: Giant Floods Don’t Just Destroy, They Renew


GARDINER, Mont. The floods that thundered from the Yellowstone Plateau and ripped through southern Montana in June changed so much of the Yellowstone River that whitewater rafting guides said they needed to relearn how to float the altered route.

The flood, fueled by torrential rains that fell on top of the melting snow, tore much of the river’s template — the river’s physical features — and built a new one. The swell was measured at more than 51,000 cubic feet per second just north of the park, far more than the peak of more than 32,000 in the 1990s flooding. It’s the kind of event that happens here once every 500 years; floods of this magnitude are known as reset floods.

Experts say the flooding, as well as the recent record-breaking floods in Australia and southern China, are likely caused by a warmer atmosphere, which can hold more moisture; and that more such floods are predicted as the world continues to warm.

Amid the damage and disruption caused by flooding rivers, experts point out that flooding plays a critical ecological role over time. They are messy and chaotic – the technical term is disruption regimes. Floods are considered an adrenaline rush for the evolutionary survival of river systems. Like the conditions caused by wildfires, nature begins to rebuild once the flames and smoke are extinguished.

“A regular flood that goes over the riverbank rewaters the landscape, rejuvenates vegetation and leaves behind a layer 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 is a fertile situation that allows a good breeding ground for poplars.”

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

“A gangbuster reset flood is a whole different deal,” said Dr. schmidt. “It tears up the oldest trees. It sends the river in a new direction. It channels the floodplains and rejuvenates everything and gives the river a new lease of life.”

Although floods destroy the natural world, said Dr. Schmidt that the post-Flood benefits far outweigh the damage as the years go by.

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

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However, this natural ecological reconstruction process is under threat.

Experts are concerned about the effects of the wave of dams on river systems, due to economic benefits. Dam building has become especially pronounced as the climate changes. Only a third of the world’s rivers continue to flow freely. (The Yellowstone is the longest undamaged river in the United States at nearly 700 miles.) Dams bring many benefits, from power generation to flood prevention, but they also have significant effects, especially those caused by the end of high water levels. downstream .

The periodic flooding and retreat of a river is called a flood pulse, and while it can cause problems for people living along a stream, it also turns rivers into biodiversity hotspots. The floodplains and the river channel form one system. A free-flowing river behaves like a fire hose over time, meanders through a landscape much wider than the main channel when it overflows, creating and feeding a rich patchwork of swamps, swamps, horseshoe-shaped lakes, river channels, ponds during depositing sediment and other debris throughout the 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 reconnecting habitat outside the channel with the main trunk of the ecosystem,” allowing fish, amphibians and other species to return to the river and exchange nutrients, 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.”

“If people build dams to control the spring surge, it has immediate damaging effects,” Mr Keddy said. “Once you turn off the spring floods, the wetlands along the entire watershed start to shrink back toward the river.”

An absence of flooding can wipe out species that have adapted to its cadence. Some fish species, such as the silver whitefish in the Rio Grande, are endangered, in part due to the lack of a flood pulse that triggers spawning. Cottonwood galleries along the river, known in the southwest as bosques, are in danger. Bosques are the tall, majestic, deciduous forests commonly found along rivers, shady desert oases that provide a habitat for a wide variety of birds, mammals, and reptiles.

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The bosque along New Mexico’s middle Rio Grande 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, creating fertile soil for the onset of the bosque. But the flood has also wiped out farms and towns. In the 1960s, construction began on the giant Cochiti Dam, 50 miles north of Albuquerque, to thwart the flow of water and sediment along the river. It worked – at a price.

The dam also ended the flood pulse, which prevented young poplars from establishing themselves, leaving only the eight-decade-old trees that grew up after the flood. Craig Allen, a retired USGS ecologist in Santa Fe, NM, calls it a “zombie forest.”

“They’re the living dead,” he said. “The whole riparian system has been transformed into something much drier.” Invasive, fire-prone tree species, such as tamarisk, have taken up residence under the old poplars. Bosque wildfires, 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 is undermining the base of the food chain,” said Matt Kondolf, a professor of land use planning at the University of California, Berkeley.

Because the dam also reduces the flow of electricity, “it simplifies the canal,” he said. “So, where you used to have grind bars and pools and riffles, it all gets washed away and you get bowling alley geometry. If there’s a fish in there, they have nowhere to hide, they’re just washed downstream.”

Floods also boost life in the main trench by capturing nutrients from the floodplain and bringing them to the main trench, providing more food for insects, which in turn feed other creatures.

After a major flood at a research site on Sycamore Creek in Arizona, Dr. Lytle thought the waterway, with fallen trees and mounds of mud, looked like a disaster. “Debris was everywhere,” he says. However, when he and his team started looking under the streambed, they found more mayflies than they had ever seen. “A big pulse of nutrition was caused by that big flood,” he said. “The conditions were just right to explode in numbers. That means more food for birds, lizards, spiders and fish.”

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Removing flooding from the landscape reduces the number of ecological niches and species, simplifying the ecosystem. Wet patches along the river are drying up – a trend accelerating climate change – and becoming less resilient.

Dams not only stop the high water pulse, they also change the water temperature. The river above the dam becomes a lake where many native fish species cannot survive and warm water fish, such as smallmouth bass, thrive. Under the dam, the natural flow regime to which many species are adapted is changing.

An ecological change that dams bring to rivers is caused by something called hydropeaking. Dams that generate electricity cause daily fluctuations in the water level because of the need for electricity. However, aquatic insects, such as caddis damselflies and mayflies, are adapted to natural seasonal floods, laying eggs just below the water’s surface. These frequent drops in water levels can dry out and destroy the eggs and eliminate insect species and make the ecosystem much less diverse.

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

For example, some dams have succeeded in allowing sediment to flow into the river below. dr. For example, Schmidt has researched the effects of spills from the Glen Canyon Dam on the Colorado River to restore gravel and sandbanks.

However, climate change can make adaptive management more difficult. This year, the artificial floods that Dr. Schmidt, limited due to extremely low water levels in the Colorado River.

“It is more challenging to maintain resilience in systems that will be hit very hard and more often by multiple types 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.”

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