The bosque alongside the center Rio Grande in New Mexico is the most important such cottonwood forest within the nation, stretching practically 200 miles throughout New Mexico.
Cottonwood seeds are borne on white cotton-like puffs — therefore the title — that sail via 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. But the flood additionally worn out farms and cities. In the Nineteen Sixties, development of the large Cochiti Dam, 50 miles north of Albuquerque, acquired underway to thwart the circulation of water and sediment down the river. It labored — at a price.
The dam additionally ended the flood pulse, which has prevented younger cottonwoods from establishing, leaving solely the eight-decade-old bushes 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, comparable to tamarisk, have moved in beneath the previous cottonwoods. Bosque forest fires, as soon as unheard-of, are widespread.
Dams additionally reduce off the gravel, silt and different sediment that rivers carry, that are used to construct new ecological options throughout a flood. Fine sediment trapped behind the dam accommodates important vitamins “and the base of the food chain is undermined,” mentioned Matt Kondolf, a professor of environmental planning on the University of California, Berkeley.
Because the dam additionally reduces stream circulation, “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.”