In 1825 when white men first saw the San Pedro River in southeast Arizona, it had so many beavers that they called it Beaver River. But they were trappers, who had already wiped out the indigenous beaver populations of the north and east to provide pelt hats for European and American gentlemen. They soon exterminated the San Pedro beavers.
Cottonwoods and willows along the riverbanks grew tall, no longer pruned into bushes by the nibbling beavers. Western Willow Flycatchers lost their willowbrush nest sites. Ranchers and farmers settled the floodplain, thinking its grassy flats would make excellent pasture and farmland. Unfortunately, the native bunch grasses (unlike prairie grass) could not withstand the pounding of hooves. Broken down, they left the land prey to erosion during rainy season floods.
Farmers’ plows also destroyed the grasses. Eroded sand and earth scoured out the river’s channel to 6-12 feet deeper than its earlier level. The water table dropped. People decided the San Pedro valley was good for little but as a resting place for migrating neotropical songbirds. The San Pedro Riparian National Conservation Area was established in 1988. Ranching and farming stopped. But the ecosystem did not rebound, and the Western Willow Flycatcher did not return.
In 1999 American Rivers, a national conservation group, listed the San Pedro as the fourth most endangered waterway in the United States. Yet a year later, it wasn’t on American Rivers list at all. The difference was beavers.
Reintroduced in 1999, the beavers got to work building a dam, which nearly doubled the local width of the river and pushed water into places that had been high and dry. They brought cottonwood and willow back toward their earlier role in the ecosystem and created willow brush for the flycatchers.
Now Western Willow Flycatchers have returned to the San Pedro.
The San Pedro River is still at risk from developments along its headwaters in Mexico, and from the water-hungry city of Sierra Vista. But Beavers have shown how quickly they can improve ecosystem health and diversity when they are allowed to resume their ancient relationship with Willow.