Giant Floods Destroy and Renew Environments

The bosque alongside the center Rio Grande in New Mexico is the biggest such cottonwood forest within the nation, stretching practically 200 miles throughout New Mexico.

Cottonwood seeds are borne on white cotton-like puffs — therefore the identify — that sail by the air.

A flood in 1941 despatched an enormous quantity of sediment down the Rio Grande and created a fertile mattress for the beginnings of the bosque. However the flood additionally worn out farms and cities. Within the Nineteen Sixties, building of the enormous Cochiti Dam, 50 miles north of Albuquerque, bought underway to thwart the stream of water and sediment down the river. It labored — at a value.

The dam additionally ended the flood pulse, which has prevented younger cottonwoods from establishing, leaving solely the eight-decade-old timber 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 mentioned. “The whole riparian system has been transformed into something much drier.” Invasive fire-prone species of tree, corresponding to tamarisk, have moved in beneath the outdated cottonwoods. Bosque forest fires, as soon as unheard-of, are widespread.

Dams additionally minimize off the gravel, silt and different sediment that rivers carry, that are used to construct new ecological options throughout a flood. Advantageous sediment trapped behind the dam comprises important vitamins “and the base of the food chain is undermined,” mentioned Matt Kondolf, a professor of environmental planning on the College of California, Berkeley.

As a result of the dam additionally reduces stream stream, “it simplifies the channel,” he mentioned. “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.”