Ghost River: Reconstructing the “Paleo-Hydrology” of the Big Bend Rio Grande
The drying of the Rio Grande in Big Bend last spring was stark evidence of its dire condition — and restoring it requires knowing something of its original state.
The drying of the Rio Grande last spring — when, for weeks, Big Bend National Park's iconic canyons framed an empty riverbed — underscored a stark reality: the river here is deeply besieged. Yet not all are resigned to its continued degradation. In recent years, diverse scientists have turned their attention to restoring the Big Bend Rio Grande.
Restoration requires knowing what has been lost. And in the case of the Rio Grande, that's harder than it might sound. Hydrologist Todd Blythe took up the task: In a quest of “paleo-hydrology,” he sought to understand how the natural river once functioned. The power and dynamism of the river he discovered are almost unimaginable.
“I'm working with a lot of the same stuff up here,” Blythe said. “It's just a different area. It's kind of shocking once you get into it professionally, how you move to a different area, and you're like, 'Well, these are the same problems all over again.'”
Blythe is a Montana native, working now for that state's natural resources department. But his graduate research was on the Big Bend Rio Grande – and the river here remains close to his heart.
His mentor was Jack Schmidt, a pioneering river scientist at Utah State University. Schmidt diagnosed how dams had transformed the Colorado River – in particular the terrain and ecology of the Grand Canyon. And his work led to “environmental flows” – the use of dam releases to mimic ancient patterns, to give river ecosystems a fighting chance. A Big Bend float trip inspired Schmidt to undertake something similar here.
Blythe was charged with reconstructing the Rio Grande's “natural flow regime.” On its journey from the Continental Divide and the Colorado Rockies through the Chihuahuan Desert, how had this river once surged and ebbed?
The first records of water levels date to the 1880s, from a gauge at Embudo, north of Santa Fe. That gauge was, in fact, the first of its kind anywhere. A scientist seeking to understand changing Rio Grande flows would seem to have plenty to work with.
“You look at it and you say, 'Wow, it's really data rich,'” Blythe said. “We have historic data going back to 1888, 1889. But when you start doing these reconstructions and trying to back-calculate a natural flow, you realize that even in the 1880s it was not natural. We don't have a shred of measured record of when the system as a whole was a natural system.”
The politics surrounding the Rio Grande were already heated by the 1880s. Farming in Colorado's San Luis Valley was diverting a majority of the river – leaving the community of Albuquerque dry, not to mention water users further south in Mexico. The Embudo gauge was installed to help manage the conflict.
It's arresting to consider – few if any white people ever experienced the Big Bend Rio Grande in its natural state. When Robert T. Hill led the first recorded expedition of the remote Big Bend canyons in 1899, he was encountering a profoundly altered river.
With historic records of limited use, Blythe gathered data on the natural “inputs” to the river, from headwaters and tributaries. And he used statistical modeling to estimate flows before large-scale diversion. The Big Bend Rio Grande, he concluded, once surged in June and July, with Rocky Mountain snowmelt. High water continued into the fall, as monsoons filled Mexico's Rio Conchos, which joins the Rio Grande at Presidio-Ojinaga.
Snowmelt floods were integral to the ecology of the Big Bend Rio Grande. The relatively cool, clear waters filled floodplains and backwaters. Fish – like the Rio Grande silvery minnow, now locally extinct – timed their spawning to the snowmelt floods. By the early 1900s, only 40 percent of natural flows reached the Big Bend from the river's main stem. After the construction of Elephant Butte Dam, in 1916, that dropped to 5 percent.
A statistical model is one thing – but how did those annual floods impact the desert wilderness and deep canyons of the Big Bend? To field test his findings, Blythe focused on Boquillas Canyon, the shining limestone gorge on the east side of Big Bend National Park.
There, above the present river channel, he found three levels of flood deposits. The first, which he called “flood sands 1,” was just a few feet up – and likely reflect the flooding of desert tributaries during monsoon rains. Ten feet higher, “flood sands 2” were likely laid down in 2008, when a tropical storm inundated the Rio Conchos and produced damaging floods here.
Then, in side canyons and on elevated terraces, Blythe found “flood sands 3” – some 30 feet above the current channel. Trying to imagine the associated floodwaters boggles the mind.
“It was fascinating researching in the canyons,” Blythe said. “You're standing on these deposits that you absolutely know the river put it there – you can tell if wind put it there, it's a different structure. We had four or five of us looking at it and confirming, 'Yes, that's water. Water deposited that here.' And you're standing up there and trying to envision what that looked like in that canyon at that point, and you're like, 'Whoa.'”
It would have taken flows of more than 150,000 cubic-feet-per-second to leave these sands behind. By comparison, the Big Bend Rio Grande saw its highest sustained flows in decades last fall – and those were below 20,000 cubic-feet-per-second.
“Flood sands 3,” Blythe said, probably date to 1904 – when the last truly massive flood occurred. In the not-so-distant past, such events were likely regular, if not routine.
The Rio Grande is what's known as an “exotic river” – one that begins in distant headwaters, and flows through desert regions. It shares that status with other fabled waterways – the Nile and Yellow Rivers, the Tigris, Euphrates and Jordan. Blythe's work shows that the Big Bend Rio Grande was shaped in important ways by the far-off snowcapped peaks of the San Juans and Sangre de Cristos.
There's little prospect flows from that upper reach will be restored here. But there are discussions about “environmental flows” from the Rio Conchos, the Mexican tributary that delivers most of the water to the Big Bend river. Conchos dam releases might someday be timed to recreate, in miniature fashion, the ancient snowmelt floods. And that could foster the conditions threatened aquatic creatures require.
It's not simple. U.S. lawmakers have mandated environmental steps for the Grand Canyon, while such measures in the Big Bend would require binational agreement. But by discovering the Rio Grande's vanished natural state, Blythe and others have provided a foundation for that.