Crash of the cottonwoods
Iconic trees decline on the West's overtaxed rivers
On a June morning, Gabrielle Katz walks a six-mile stretch of Arizona's lower San Pedro River called Three Links Farm and counts the plants growing there. The diminutive Appalachian State University geographer pays special attention to the cottonwoods, common fixtures of Western streams and rivers that figure prominently in Native American legends and pioneer accounts. Because they need seasonal floods and access to groundwater to reproduce and thrive, the trees have a lot to say about the health of the river and its water supply. Here, cottonwood seedlings dot the riverbed and a few adolescents rise from the shores and sandbars alongside older trees. The mixture suggests that natural flows are intact and the river is doing well.
But on drier reaches, the trees tell a different story. A century of overgrazing, logging and pumping for agriculture and urban development has depleted and reshaped the San Pedro -- killing mature cottonwoods in some areas and preventing the germination of new seedlings to replace them. The river owes its healthy stretches to "an amazing array of projects and policies‚" put in place to preserve it, says Holly Richter, director of The Nature Conservancy's upper San Pedro program. "But there's still more to do,‚" she adds, and as declining cottonwood stands on rivers across the West show, restoration here and elsewhere will be an uphill battle.
Streamside stands of cottonwoods, willows and other trees comprise less than 2 percent of the land area in the Western U.S., but they do essential work anchoring riverbanks, stabilizing soil and filtering much of the West's water. Cottonwoods shade and cool insects and larger animals, and they offer migratory birds dependable habitat from Mexico into Canada, according to U.S. Geological Survey ecologist Michael Scott.
The resilient trees can adapt to harsh conditions and fluctuating water supplies, says Scott, who has studied and admired them for 20 years. Even so, a slew of factors is killing off the trees and endangering the habitat they provide. Dams and diversions for hydroelectric power, agriculture and urban development alter or eliminate the seasonal floods the trees depend on, and often salinize the water. When floodplains are cleared for agriculture or development and riparian areas are overgrazed, the trees' vulnerable seedlings face erosion, trampling livestock and hungry browsers. And pumping groundwater lowers the water table below the cottonwoods' roots, further shrinking the trees' chances for survival.
Each of these forces has acted on the Truckee River in California and Nevada, says cottonwood research pioneer Stewart Rood of the University of Lethbridge in Alberta. Since the early 1900s, the river has been straightened, dammed and diverted for agricultural and municipal use, and by the early 1990s, it had lost approximately 90 percent of its cottonwoods and 42 bird species between Lake Tahoe in California and Nevada's Pyramid Lake. The Saint Mary River, which flows from Montana's Glacier National Park into Alberta, lost 95 percent of its cottonwood stands after damming abruptly ended its spring floods in the 1950s. From the badlands of North Dakota to the Milk River floodplain of northern Montana, cottonwoods along the vast Missouri River, home to the threatened bald eagle and other riparian fauna, are also slowly disappearing.
Climate change further stacks the odds against the trees. A study published in the journal Science last spring predicts that global warming will prolong the West's current drought indefinitely. The combination of diminishing water, drier seasons and rising temperatures will render riparian areas more susceptible to intense fires. Invasive species like tamarisk will thrive, because they cope better with high-intensity fires and saline water.
As the cottonwoods die, they feed the cycle of destruction: Sunlight filters through the holes in forest canopy left by dying trees, drying the soil below and fueling the growth of non-natives and the likelihood of fires.
"If it were climate change alone, I wouldn't worry for cottonwoods, because they've shown such adaptability,‚" says Scott. "The problem comes when you throw climate change into the mix of all these other changes we've made.‚"
The combination of factors makes restoring cottonwood habitat hard. "Because changing water management is too controversial, people try to just plant them,‚" says Scott, and many efforts fail.
Restoration efforts that do tackle the underlying causes of riparian decline have more success. On the Truckee, a water-management system that alters the river's water level gradually to mimic natural flow patterns -- a system partly based on Stewart Rood's models -- more than doubled the river's cottonwood cover in places. It also helped preserve Pyramid Lake's endangered cui-ui fish and the threatened Lahontan cutthroat trout.
Three Links Farm is another lucky place. After a century of clearing and logging stripped the river's shores of cottonwoods and other plants, The Nature Conservancy purchased the land in 2002, removed roads, fences and grazing animals, cut agricultural water use by 90 percent, and let the river recover on its own. Katz's counts monitor the project's success, and although her findings are not yet conclusive, the cottonwoods are making a slow return, says Conservancy project manager Barbara Clark, along with southwestern willow flycatchers, migratory songbirds listed as endangered in 1995.
It took the prospect of losing the cui-ui and the flycatcher to garner support for those restoration efforts, and that, according to Scott, may be the greatest obstacle for a familiar tree like the cottonwood.
"Our laws are such that something has to be rare and endangered for us to recognize its value,‚" he says. "We ignore the value of the common.‚"
The author is a freelance writer and photographer in Tucson, Arizona.
This article was made possible with support from the William C. Kenney Watershed Protection Foundation and the Jay Kenney Foundation.