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 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 large Cochiti Dam, 50 miles north of Albuquerque, acquired underway to thwart the move 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 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 residing useless,” he stated. “The entire riparian system has been remodeled into one thing a lot drier.” Invasive fire-prone species of tree, akin to tamarisk, have moved in beneath the outdated cottonwoods. Bosque forest fires, as soon as unheard-of, are frequent.
Dams additionally minimize off the gravel, silt and different sediment that rivers carry, that are used to construct new ecological options throughout a flood. Positive sediment trapped behind the dam comprises important vitamins “and the bottom of the meals chain is undermined,” stated Matt Kondolf, a professor of environmental planning on the College of California, Berkeley.
As a result of the dam additionally reduces stream move, “it simplifies the channel,” he stated. “So, the place earlier than you had gravel bars and swimming pools and riffles, all of that will get washed away, and also you wind up with a bowling alley geometry. If there’s a fish in there, there’s no place for them to cover, they only get washed downstream.”