Huge floods destroy and rebuild environments

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

Poplar seeds are carried on white, cotton-like puffs—hence the name—that soar 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 began on the massive Cochiti Dam 50 miles north of Albuquerque to prevent the flow of water and sediment down the river. It worked – at a price.

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

“It’s the living dead,” he said. “The entire riparian system has turned into something much drier.” Invasive fire-susceptible tree species like the tamarisk have taken up residence under the old cottonwoods. Bosque wildfires, once unheard of, are rampant.

Dams also cut off gravel, silt, and other sediment that rivers carry with them, 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 being undermined,” said Matt Kondolf, a professor of environmental planning at the University of California, Berkeley.

Because the dam also reduces current flow, “it simplifies the canal,” he said. “So where there used to be gravel bars and pools and ripples, all of that gets washed away and you end up with a bowling alley geometry. If there’s a fish in there, there’s nowhere for it to hide, it just gets washed downstream.”

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