NATURE NOTES:
Nowhere else to go: the Rio Grande goes dry, and aquatic life hangs in the balance

The Rio Grande – like all the world’s great rivers – meets many needs, and means many things. It’s water for drinking and farming – a foundation for societies past and present. It’s a place of beauty, and a resource for recreation and tourism. For 175 years, it’s been put to political use, as a border between nations. 

But for the river’s aquatic life – the fish, the mussels, the turtles – the Rio Grande is home. What does it mean for these communities when the river goes dry, as it has in much of Big Bend this spring? And how has the human-driven degradation of the river impacted these creatures, who have nowhere else to go?

This spring, perhaps 75 miles or more of the Rio Grande in Big Bend National Park have been reduced to a dry riverbed, with isolated pools. It’s stark. But it’s not unprecedented. 

It happened in the 1950s, amidst historic drought, and briefly during the drought of 2011. But in 2003, the gauge at Johnson Ranch – in the remote river stretch called the “Great Unknown” – registered zero or near-zero flows for 58 days straight. 

Dr. Bruce Moring is an eco-hydrologist, who studied the impacts of that 2003 “dewatering.” 

“The actors there were the kind of fish you’d expect to find in a hydrologically stressed place in the river like that,” Moring said, “catfish, gar, carp. It was interesting that we didn’t pull up one Tamaulipas shiner, one red shiner – and we seined the heck out of it, in 2004.”

In 1999, Moring had conducted baseline fish surveys at five sites in Big Bend. At Johnson Ranch, he found several native catfish species and the non-native Asian carp. But there were also smaller natives – the Tamaulipas shiner, the Rio Grande shiner, the red shiner, the river carpsucker.

In 2004, water had returned to Johnson Ranch. But the fish diversity had not.

Healthy rivers are wondrously complex – with a host of what ecologists call “meso-habitats.” Backwaters and riffles, runs, eddies and pools – different fish are adapted to each of these niches, to live, feed and reproduce.

And shiners, and other minnow-type fish, are “swift-water” specialists. Obviously, such habitats vanish when a river goes dry. Catfish and gar can endure in isolated pools – and the swift-water specialists become easy prey. 

“They’re definitely fish that utilize those runs and riffles,” Moring said, “and those are some of the most impacted at low flow conditions. Those fish are adapted to that, and those guys that can hang out and make it in the pools – of course, if you’re a shiner, you went from a riffle to no flow, and you’ve got two pools at the end of the riffle. You’re probably fodder at that point for everybody that’s left.”

Images of a dry Rio Grande are arresting. But they’re only an acute manifestation of a broader crisis.

With heavy upstream diversion, little water has reached the Big Bend from the Rio Grande’s main stem for a century and a half. Flows here depend on the Rio Conchos, a Mexican tributary, and when the Conchos is in drought, as now, the river here can go dry.

The end of the ancient flow patterns has impacted aquatic life. Four of the 36 fish species once native to the Big Bend are locally extinct. That includes the Rio Grande silvery minnow.

The minnow is a “pelagic spawner” – historically, these fish released their sperm and eggs during annual snowmelt floods – and let the surging floodwaters fertilize and disperse those eggs. They struggled to reproduce in the absence of the spring floods. And the dewatering of the 50s likely dealt them their final blow here.

Fish are remarkably resilient. Moring pointed to the Trinity River, where native fish rebounded after the city of Dallas stopped releasing untreated wastewater into the river. And in the Big Bend downstream of Rio Grande Village, spring flows help sustain aquatic life.

But there’s a limit to the resiliency. If this year’s conditions become routine, the effects in parts of the Rio Grande could be permanent.

“I suspect you’d see some reaches of the river would be slow if ever to recolonize with fish,” Moring said. “Some would get back in there, the carp and gar, but the community would really be impacted. If it occurs with enough frequency, if it comes in back to back years, it could really have a major impact.” 

On next week’s Nature Notes, we explore research into how the river’s aquatic life might be preserved.