The Troubled Rio Grande
Mud and giant cane dominate the Rio Grande's banks at the entrance to Colorado Canyon. (Photo by Megan Wilde)
By Megan Wilde
On a sunny November day in Big Bend Ranch State Park, the Rio Grande roils swiftly around a muddy bend into Colorado Canyon. Dragonflies bask on rocks, and a breeze ripples through giant cane on the banks. Looking at this idyllic scene, you might not think anything’s wrong with the Rio Grande. But in our region, the river isn’t really doing its job. What’s going on?
Jeff Bennett is Big Bend National Park’s hydrologist.
Bennett: Luna Leopold said that a river’s job is to move the mountains to the sea. And the Rio Grande is having a little bit of trouble doing that right now.
As rain falls on Big Bend’s mountains, grasslands and desert, rainwater picks up bits of eroded earth. Those bits are called sediment, and range from flecks of mud and grains of sand, to larger pebbles and rocks. This sediment washes into our creeks and streams, which in turn carry it to the Rio Grande.
In its natural state, the river eventually flushed sediment all the way into the Gulf of Mexico. But today’s Rio Grande is far from natural. It’s highly regulated and water-deprived, with almost every drop controlled by dams on both sides of the border. This altered river simply isn’t able to do its job very well. So in our region, sediment is piling up at tributary mouths and slowly filling the river channel.
This sediment overload is essentially choking the Rio Grande, leaving less room for water in the river channel. Now, when there is a surge in water flow—after heavy rains or water releases from upstream dams—the river overflows its banks and inundates the floodplain. Bennett says that’s why floods are becoming more frequent and serious, like the one that deluged Presidio, Ojinaga and other Mexican communities downstream in 2008.
But flooding isn’t the only problem. Salt cedar and giant cane can colonize a moist backlog of sediment in a matter of weeks. Once established, these exotic species are incredibly difficult to remove. And they’re extremely good at anchoring sediment, making it even harder for the river to flush sediment downstream.
Most rivers actually have the opposite problem. Dams deprive them of sediment, so river banks are washed away. Of course, many dams also hold back water on the Rio Grande, as well as on the Rio Conchos in Mexico, where most of the Rio Grande’s water comes from in the Big Bend region. But the nearest dam on the Conchos is hundreds of miles away, so Bennett says plenty of tributaries between there and here dump sediment into the Rio Grande.
Bennett: This is a unique problem that doesn’t exist everywhere and it will require some unique solutions.
Thankfully, one possible solution is in sight. Dam operators in Mexico currently release water at their convenience and to meet the needs of irrigators and treaty obligations, not to provide any ecological benefit. Bennett and other water managers in the U.S. think that changing how water is released from dams on the Conchos might help the Rio Grande. They recently began discussing this possibility with Mexican authorities.
Bennett: We’re not suggesting that they release more water. We’re just suggesting that the water they do release they release in such a way to move sediment downstream. Or at least to move it out of the mouths of some of the tributaries.
How that should happen is still being explored, and discussions with Mexican authorities have only just begun. But Bennett is hopeful that with a little help, the Rio Grande can someday get back to moving mountains to the sea.
Have a question or comment about this episode? Contact Nature Notes Coordinator Megan Wilde at mwilde [at] cdri [dot] org. Or discuss this episode on Nature Notes’ Facebook page. This episode originally aired on November 18, 2010.
References & Resources for Educators
- The Rio Grande: Big Bend National Park
- Out of the Blue: K-12 Hydrology and Water-Management Activities and Lesson Plans from Great Sand Dunes National Park
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Nature Notes is sponsored by the Meadows Foundation and the Dixon Water Foundation and is produced in cooperation with Marfa Public Radio.
