The bosque along the middle Rio Grande in New Mexico 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 and created a fertile bed for the beginnings of the bosque. But the flood also wiped out farms and towns. In the 1960s, construction of the giant Cochiti Dam, 50 miles north of Albuquerque, got underway to thwart the flow of water and sediment down the river. It worked — at a cost.
The dam also ended the flood pulse, which has prevented young cottonwoods from establishing, leaving only the eight-decade-old trees 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 said. “The whole riparian system has been transformed into something much drier.” Invasive fire-prone species of tree, such as tamarisk, have moved in beneath the old cottonwoods. Bosque forest fires, 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 the base of the food chain is undermined,” said Matt Kondolf, a professor of environmental planning at the University of California, Berkeley.
Because the dam also reduces stream flow, “it simplifies the channel,” he said. “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.”