Beavers, through their assiduous dam building, can recharge groundwater and provide habitat for fish and wildlife. In the Pacific Northwest, for example, managers are bringing back beaver as part of trout and salmon management.

“God bless beavers and their industrious nature,” writes Trout Unlimited’s Idaho-based Chris Hunt in Hatch magazine. “They make habitat for the fish we love, and opportunities to catch them.” True enough, in Idaho.

But the notion, ubiquitous in America, that all beavers everywhere are a panacea for what ails an ecosystem is misinformed. Yes, beavers are beneficial – in the right places.

In the wrong places – watersheds degraded by humans – they’re a scourge. The environmental community and the public tend to have trouble grasping these two realities simultaneously.

In his essay “Thinking Like a Mountain,” Aldo Leopold, father of wildlife management, described how killing wolves for the supposed benefit of deer resulted in obliterating deer habitat along with the deer themselves. Everything he wrote about deer applies equally to beavers. Both species depend on essentially the same forage – in unnatural abundance because of massive logging – and the main predators of both are wolves that are no longer around.

Beavers affect native ecosystems the way red wine affects human bodies: One glass a day helps the heart; 20 blows out the liver.

In the wrong places, beavers grossly overpopulate, blocking trout migration, stripping streamside cover, choking spawning gravel with silt and muck, and converting oxygen-rich streams to deadwater. That’s because humans have eliminated wolves and old growth from most of the West, and stream corridors now grow willow and aspen – beaver candy.

Consider the debacle in Nevada. This from Kim Toulouse, the Nevada Department of Wildlife’s former conservation educator: “Historically, virtually every stream in the northern half of Nevada held some form of cutthroat trout. Additionally, many small-order streams also held native redband and bull trout. When the push started (for trout recovery) we discovered that many single-order streams were infested with heavy populations of beavers. 

“Extremely high numbers of beaver dams on these systems led to loss of gene flow and precluded the ability of fish to move up and down these systems. Additionally, fish found it difficult to find suitable spawning grounds due to heavy siltation caused by the dams. The loss of riparian habitat led to erosion, more siltation, less shade, higher water temperatures, loss of native riparian vegetation, and establishment of noxious invasive plants.”

So Nevada initiated major beaver control. But politicians, incited by the Humane Society of the U.S., shut it down.

Beaver damage to Minnesota and Wisconsin trout streams is even worse. Fisheries managers have to hire Wildlife Services, a federal agency, to trap beavers and blow up dams. It’s expensive, so only a small percentage of streams can be salvaged.

And Trout Unlimited reports that in Minnesota’s Knife River watershed, “artificially high beaver numbers … threaten the survival of coldwater fisheries, as well as the health of the watershed and Lake Superior.” But an outfit ironically called “Advocates for the Knife River Watershed” is fighting to nix beaver control, circulating junk science and such fictions as “beaver have been totally eradicated in the whole Knife River valley – over 200 square miles.”

California’s Silver King Creek watershed is the only refuge for threatened Paiute cutthroat trout, yet overpopulated beavers block migration and destroy habitat. It got so bad in Four-Mile Creek that Trout Unlimited volunteers had to reroute the stream. 

“The biggest problem I see is that beavers move into an area that doesn’t have enough forage, and they abandon their dams,” said retired state fisheries biologist Bill Sommer. “When beavers leave, the dams blow out and that causes erosion.”

Aldo Leopold could grasp two realities about deer simultaneously. Were he still alive, he’d applaud Phil Monahan, who wrote this in Trout Unlimited’s Trout Magazine: “Many anglers see the beavers’ work as predominately destructive – turning a babbling trout stream into a slow-moving wetland, for instance. Wildlife biologists recognize that each of these ‘destructive’ effects has a flip side: situations in which that very same effect is beneficial to trout. 

“After looking at all the data, then, the question, ‘Are beavers good or bad for trout streams?’ can be answered only with a definitive: ‘It depends.’”

Ted Williams is a contributor to Writers on the Range, writersontherange.org, an independent nonprofit dedicated to spurring conversation about the West. He writes for several national publications about wildlife.