The bosque along New Mexico’s middle Rio Grande 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, creating fertile soil for the onset of the bosque. But the flood has also wiped out farms and towns. In the 1960s, construction began on the giant Cochiti Dam, 50 miles north of Albuquerque, to thwart the flow of water and sediment along the river. It worked – at a price.
The dam also ended the flood pulse, which prevented young poplars from establishing themselves, leaving only the eight-decade-old trees that grew up after the flood. Craig Allen, a retired USGS ecologist in Santa Fe, NM, calls it a “zombie forest.”
“They’re the living dead,” he said. “The whole riparian system has turned into something much drier.” Invasive, fire-prone tree species, such as tamarisk, have taken up residence under the old poplars. Bosque wildfires, 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 is undermining the base of the food chain,” said Matt Kondolf, a professor of land use planning at the University of California, Berkeley.
Because the dam also reduces the flow of electricity, “it simplifies the canal,” he said. “So, where before you had grind bars and pools and riffles, that all gets washed away and you get bowling alley geometry. If there’s a fish in there, they have nowhere to hide, they’re just flushed downstream.”