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Giant floods destroy and renew environments

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

Poplar seeds are carried on white cotton-like puffs – hence the name – that float through the air.

A flood in 1941 sent a huge amount of sediment into 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 began on the giant Cochiti Dam, 50 miles north of Albuquerque, to thwart the flow of water and sediment into the river. It worked – at a cost.

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

“It’s the living dead,” he said. “The entire riparian system has been transformed into something much drier.” Invasive fire-prone tree species, such as tamarisk, have taken hold under the old poplars. Bosque wildfires, once unknown, are common.

Dams also cut off gravel, silt and other sediment carried by rivers, which are used to create new ecological features during a flood. The fine sediment trapped behind the dam contains essential nutrients “and the base of the food web is mined,” said Matt Kondolf, professor of environmental planning at the University of California, Berkeley.

Because the dam also reduces the flow of the stream, “it simplifies the channel,” he said. “So where you used to have gravel bars and pools and rapids, it all gets washed away and you end up with bowling alley geometry. If there’s a fish in there, there’s no place to hide, it’s just swept away by the current.

nytimes Eur

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