From Mighty to Muted: the Secret History of the Big Bend Rio Grande
Big Bend residents who've taken visitors to the Rio Grande have heard some version of the wisecrack: “What's so 'grand' about this river?” You explain that flowing water – in any quantity – is the pearl of great price in the desert. You take your visitor boating through the Big Bend canyons – those sanctuaries of white rock and shadow – which answers any questions about scenic grandeur.
But your outlander isn't entirely off-base.
The Rio Grande, in the words of Tennyson's “Ulysses,” is “not now that strength which in old days moved earth and heaven” – it's not the same stream that earned names like “the big river,” “the wild river of the north.” Of the water that once flowed through the Big Bend reach of the Rio Grande, only about 5 percent remains. The “Great River” has been transformed, by human needs, and choices.
Now, the U.S. Geological Survey, in partnership with the National Park Service, Sul Ross State University and others, has published a concise diagnosis of that transformation. The “fact sheet” distills years of research by hydrologist David Dean. And it points toward what might be done, if not to restore, than to rehabilitate this epic waterway.
“Historically, in any given year you had high water, potentially, for six months of the year,” Dean said. “But every four years you had a really big flood. So that was able to really maintain that wide organization of the channel.”
Dean began his research in 2006, as a graduate student at Utah State University. His adviser was Jack Schmidt – a pioneer in the science of river “geomorphology.” Dean sought to apply the approach Schmidt developed on the Colorado and other Western rivers to the Rio Grande.
His first connection was with then-Big Bend park hydrologist Jeff Bennett. Bennett shared archival park photos that put the river's changes into stark relief. They showed a braided and sprawling Rio Grande, with a wide, sandy floodplain. At the same places, the river had become a “vegetated tunnel” – a narrow “ditch” of water, hemmed in by thickets of mostly non-native vegetation.
Understanding the mechanisms behind that change, and its timeline, required integrating archival and field research.
“It's almost like a journalistic approach to things,” Dean said. “You just want to gather as much information as you possibly can to put together a story, and it helps if you have pieces of information that show you a picture over a large spatial scale, but also pieces of information that are very detailed in time.”
Dean first gathered all the aerial photos he could find – dating back to the 1930s – which provided the spatial perspective. The USGS and the International Boundary and Water Commission have maintained stream-flow records for a century, which offered temporally precise information on changes in flow and river topography.
Like other rivers with Rocky Mountain headwaters, the Rio Grande historically swelled in late spring, as snowmelt coursed down its main stem. But unlike other Western rivers, the Rio Grande here had a second pulse – when summer monsoons in Mexico's Sierra Madre filled the Rio Conchos, which joins the Rio Grande at Presidio-Ojinaga.
The construction in New Mexico of Elephant Butte Dam, in 1916, effectively ended snowmelt floods. And La Boquilla Dam was built on the upper Conchos in 1915. But Dean found that it wasn't until the 1940s, when drought conditions intensified across the West, that major changes began on the river here. Summer floods diminished. The construction on the Conchos of the Luis Leon Reservoir, in 1967, sealed the deal.
Sustained flooding now occurs only when Pacific storms fill Conchos reservoirs, forcing emergency releases. Big floods once occurred every few years. There have been only five such floods in the last 80 years.
Why does that matter? Well, consider a resource in which the Chihuahuan Desert abounds: dirt.
The Big Bend tributaries that meet the Rio Grande – Alamito, Terlingua and Tornillo Creeks, San Carlos and other creeks in Mexico – may be bone-dry most days. But when they flash, they carry massive loads of silt, clay and sand.
Large floods once moved that sediment downstream. But in the absence of such “channel-resetting” floods, the Big Bend reach is choked, by its “sediment surplus.”
That choking happens with stunning rapidity. With backhoes, Dean and collaborators dug two large trenches in park floodplains. Much of the floodplain was very young.
“We found that 3 meters, or 10 feet of sediment, had accumulated within what used to be the river channel within 17 years,” Dean said. “That was really the eye-opening part, is just how fast these changes occur on the Rio Grande.”
To further quantify the matter, Dean installed sediment-monitoring devices at two spots along the river.
Flash floods on Terlingua Creek in June 2013 provide an example. These flashes, the monitoring showed, delivered 118,000 metric tons of sediment to the river. How much sediment is that? Imagine a line of dump trucks 30 miles long, with each truck waiting to discharge a full load into the river.
Sediment accumulation initiates a cycle. Vegetation – especially invasive non-natives like giant river cane and salt cedar – establishes itself, which in turn locks sediment in place, further narrowing the channel.
“So it's kind of a feedback loop,” Dean said, “that just continues until the river gets so small it can't get any smaller. We've never seen how small that is. It's pretty small though.”
The costs in biodiversity are steep. Fish like the Rio Grande silvery minnow need slow-moving backwaters to reproduce. As the river changed from a variegated stream into a narrow channel, such habitat was lost. Many native aquatic species have vanished, or been driven to the brink.
Humans, too, pay a price. When the Rio Grande had a wide floodplain, the flooding river had room to spread out. Now, when floods do occur, the river leaps its narrow channel, inundating homes and businesses – as it did in Ojinaga in 2008, during the last big flood.
It's unlikely that channel-resetting floods will become regular occurrences again any time soon. But Dean and others have laid the groundwork for “environmental flows.” Releases from reservoirs in Mexico could be managed to ameliorate the river's sediment surplus.
“What needs to be stressed is that the community that's seeking for an environmental flow program on the Rio Grande, they're not asking for more water from anywhere,” Dean said, “just a re-operation of the way things are currently done. You could either have a very low flow occur for a really long time, or you could have a much higher flow that occurs for a shorter time. You can actually just change the dynamics of how that water is released, in order to provide some sort of environmental benefit.”
“Though much is taken, much abides,” as the Tennyson poem has it. The Big Bend canyons remain spectacular and wild. And pockets of biological richness endure – especially where springs supplement the river. But the future of the Rio Grande is largely in human hands.
The USGS fact sheet can be found here: