Rio Grande restoration conundrum
By MEGAN WILDE / The Big Bend Gazette (December 2008)
For most of its history, the Rio Grande was a mercurial wanderer through the Big Bend region, changing course with each annual flood. Its shallow waters swelled, stretched and shifted freely across low sand and gravel banks, which were populated only sparsely by cottonwoods, willows and other native plants. In a deluged whim, it could sculpt slender islands from its floor, twist into eddies, or carve oxbows into its edges. It was wide, open and wild.
But during the past century, the Rio Grande has been dammed and diverted by the two countries that manage it. Its water flow has been diminished, its flood cycles disrupted, its channel narrowed and deepened by mud and silt, and its banks choked by dense stands of exotic plants. The old river, along with many of the plants and animals adapted to its more unruly ways, is essentially gone, like a deceased relative known to us mostly from old photographs.
“Basically the river is shaped now like an irrigation ditch,” says David Dean, a Utah State University watershed science graduate student who has been studying the river’s transformation.
That transformation, to many people, seems ecological injustice, and it has repeatedly earned the Rio Grande a place on national and international most-endangered-rivers lists. It has also caused more menacing floods, such as the one this fall, Dean says.
“I think that it’s tough to accept the state of a river that we know we directly impacted, at the cost of a truly unique ecosystem,” he says.
And so, for the past five or six years, several non-profit groups, government agencies, and academic researchers have been looking at ways to rehabilitate that Rio Grande ecosystem in the Big Bend area. In mid-November, the World Wildlife Fund gathered a few dozen representatives from 25 of these groups at a workshop in Alpine to discuss what they’ve done, and to begin tackling the million-dollar question: with the river so drastically altered, and its water so tightly controlled, how far can we turn back the clock?
Workshop attendees agreed it’s a remarkably challenging problem, nuanced by a lot of scientific uncertainty and complicated by the delicate politics of two nations sharing a water-stressed river. Even still, these resource managers hope that through a combination of small-scale projects and big-picture water management plans, parts of the Rio Grande may someday resemble the river’s former self.
“We’re definitely not trying to take the river back to the way it was before the dams were in, but we’d like it in better condition than it is today,” says Mark Briggs, Big Bend coordinator for the World Wildlife Fund. “It’s a very difficult objective, but I think we came a long way [during the workshop].”
That sentiment was echoed by Raymond Skiles, chief of science and resource management at Big Bend National Park.
“Of course we’re not in a position to snap our fingers and have everything back the way it was,” Skiles says. “At the same time, we recognize down here in the park we do have some issues we have control over.”
Those issues are salt cedar and giant cane, he says, and so far, most river rehabilitation projects in the national park and elsewhere in the region have focused on eradicating patches of these invasive plants.
The altered river is thought to excel at watering these weeds, which now cover vast swathes of Rio Grande floodplain. These plants’ exoticness is only part of the problem, particularly given that even native plants probably belonged in only a few patches alongside the river, Skiles explained. While salt cedar and cane don’t provide ideal habitat for many animals, the bigger issue is how they are thought to exacerbate the river’s current condition.
“Rivers are not just rivers of water. They’re rivers of sediment and rivers of nutrients as well,” says Jeff Bennett, the national park’s hydrologist.
As the river passes through stands of cane and salt cedar, the water slows down and drops those sediments and nutrients. These deposits build up the river’s banks, raising the floodplain and thinning the channel. In turn, the built-up banks provide more places for salt cedar and cane to flourish.
The exotic plants also act like armor, protecting the narrowed channel and elevated floodplain. Historically, annual floods served as a sort of riparian Roto-Rooter. By regularly scouring away mud, silt and vegetation, floods kept the river channel shallow and wide, and maintained its gently sloping banks and low, open floodplain. But dense stands of cane and salt cedar lock mud and silt in place, so even when there are floods now, the inundation’s cleansing power is diminished.
Resource managers hope that removing cane and salt cedar will destabilize the banks and allow smaller floods and the occasional big flood to do their job, moving mud and silt downstream and helping the river resume a more natural form. That natural form, Skiles says, will hopefully include a river floor mosaic of cobbles, gravels and sands—the varied habitat that could once again support a diverse array of native fish, mussels and other critters, which now are largely missing from the Rio Grande.
Eradicating exotics and opening the banks may also help native plants establish themselves, says Joe Sirotnak, the park’s botanist. While it’s now thought the river’s banks were only sparsely vegetated in the past, national park staff are planting natives in some places where exotics have been removed. Without replacing the exotics, cane and salt cedar will probably return, explains Sirotnak.
“With the current hydrology of the river, something is going to grow there,” he says. He would rather that growth be something native—such as huisache, mesquite, cottonwood, willow, or wildflowers—that will provide better habitat for most species and hopefully won’t armor the banks as much.
Recent projects at Boquillas Canyon, Hot Springs, and further upstream have used fire, herbicides, salt cedar-eating beetles, and old-fashioned tree felling to denude a few river miles of exotics. So far, the results have been promising, particularly after the flood this fall. But Briggs and park staff caution these projects’ successes are still being evaluated, and there remains some uncertainty about the relationship between exotics and the river’s condition.
“We don’t have a good handle on whether changing [the river’s] flow is going to alter the competitive balance between natives and exotics,” Sirotnak says. “But the hypothesis is that it will.”
It’s also unclear, he says, whether the flood this fall would have stripped these areas of salt cedar and cane even without the eradication projects. Certainly, though, the flood has provided an opportunity to experiment with maintaining the river’s width and keeping project areas free of exotics.
Of course, getting rid of salt cedar and cane from all reaches of the Rio Grande would be nearly impossible, and projects currently underway in this region have much more modest aims. Looking up from these small-scale projects and seeing what work remains can still be daunting. Sirotnak described feeling like Don Quixote tilting at little windmills when he considers the hundred or so miles of river the park manages.
“We’d be lucky if we could actually work on and get two or three miles of that river rehabilitated, which doesn’t mean restored. It just means functioning,” he says. He compared river rehabilitation to rehabilitating a stroke victim; you might be able to help them walk again but they may never be the same person.
“Without a basin-wide approach, it’s a giant windmill that you’ll never knock off,” he went on. “But there are little victories.”
The issue with rehabilitating the entire basin – the giant windmill which no one is sure how to tackle yet – has to do with the river itself. Exotics are a problem that, as Skiles says, park staff and other resource managers can control locally to some extent. But the river’s real affliction seems to be its altered hydrology and geomorphology – how much water is in the channel; when that water flows; and how that water shapes and moves the land cradling and surrounding the river.
There are some things we don’t know about the Rio Grande’s needs in this regard, and about how exactly dam building and irrigation diversions have affected the river system, according to park staff and researchers. But there is also a lot we do know, Skiles says. We do know there is less water in the river than there used to be, and that how water is released from dams does impact what the river looks like, according to Bennett. For example, he says in spring 2006 Mexico released water from dams into the Rio Conchos, which flows into the Rio Grande above Presidio; the long, low water springtime flow coincided with an enormous growth of salt cedar here in the Big Bend.
We also know, Bennett says, that the highest flows of the year—those scouring floods—have decreased by about half in recent decades. And, adds Dean, scouring floods now happen less often—almost every decade instead of almost every year. Furthermore, Dean says last year the river was less than half as wide as it was in the 1940s. That narrowing, combined with the other alterations in the river’s water supply, seems to have had a tremendous impact on the river’s velocity, depth, and shape, as well as how it floods.
“The flood we just experienced this September had, I believe, the highest flood stage”—that is, the highest water level—“of any other flood recorded in the past. But the volume of water that passed down the river was typical,” Dean says. In other words, the narrower river couldn’t hold as much water, so what was really a moderate amount of flood water more quickly filled and overflowed the channel.
All of these changes in the river’s hydrology and geomorphology seem to keep the feet of salt cedar and cane perfectly wet. The exotics thrive and exacerbate the channel narrowing, which exacerbates the growth of exotics, and so on. The giant windmill looms larger. Even if the entire floodplain could be stripped of exotics, or if the entire channel could be dredged into pre-1900 form, the river no longer seems to have the water it needs, when it needs it, to maintain its former shape.
But with gaps in our understanding of the river’s natural hydrology and geomorphology, and of how alterations in the current water supply might affect the river system, researchers and resource managers are still trying to determine how best to approach a basin-wide rehabilitation.
“We can have a great vision for the river,” Sirotnak says, “but the river has to maintain itself. The water is going to do that. The difficulty is going to be determining how much and when. Then there’s the whole sociopolitical thing.”
Ultimately, any decisions about how much water is in the river and when it’s released will be up to dam operators and water managers in the United States and Mexico. And in the case of the Big Bend region, where the Rio Grande’s water mostly comes from the Rio Conchos, it is primarily Mexico that controls the tap. With most every drop of river water governed by binational treaty and allocated to riverside landowners, resource managers would like but don’t expect to get much more water from that tap. Fortunately, that doesn’t seem to be the solution anyway.
“Just because of the condition of the river, if you simply put a little more water in the river, it isn’t going to fix things,” Briggs says.
Instead, Briggs and park staff say they hope to see if there is some flexibility in when and how Mexico releases water into the Conchos, and thereby the Big Bend reach of the Rio Grande.
“The Conchos drives the hydrology of the river,” Briggs says. “So any major change in terms of river flow—tweaking it a little bit so it is a little more geared toward the natural hydrology—will have to come from the Conchos and from Mexico.”
“That needs to be pursued in a very sensitive manner,” he went on. “It has to be a very binational team.”
Just such a binational team was assembled at the workshop in Alpine last month, and for the first time as a group, they began discussing their vision for what a rehabilitated Rio Grande might look like.
“It’s an extremely important step to be developing a broad partnership and shared vision. That’s what that meeting to me was all about,” Skiles says. “Until that conference, we didn’t have a common understanding of where we’d been and where we’re going. That helped us take a few baby steps forward.”
“How far can we go? That’s far from being determined,” he continued. “Everyone would agree that we can’t turn the clock back to the 1940s. But we do know that a little bit helps. We know that we can make a difference.”