In October 1916, dignitaries gathered at the Rio Grande, near present-day Truth or Consequences, New Mexico, for an event said to be “of world-wide importance in the irrigation field.” It was the dedication of Elephant Butte Dam, then the largest impoundment reservoir in the world.
It would store the river's spring floodwaters. By regulating flows, it promised to “make the desert bloom” from Southern New Mexico to El Paso and Juarez.
But the dam also triggered a chain of unintended consequences. The riverside habitat was transformed. A century later, scientists are still tracking these sweeping changes.
The Chihuahuan Desert is outstanding in many respects. For example, it's got a lot of sand and soil. There's not much vegetation to hold soil in place. When rains pour into hills and canyons, side channels move tremendous amounts of sediment into the Rio Grande.
The Rio Grande requires powerful flows to move that sediment down to the Gulf of Mexico. When Elephant Butte was dammed up, the annual springtime floods – fueled by snowmelt from the Rocky Mountains – came to an end. And sediment began to accumulate in the river.
Dan Cadol is a professor of hydrology at the New Mexico Institute of Mining and Technology in Socorro.
“But when we block that off, all of that sediment in transport gets trapped in the reservoir and slowly fills it up,” Cadol said. “In the case of the Rio Grande, about a quarter of the reservoir capacity has been filled up by sediment over the last 100 years, since it was closed in 1916.”
Even downstream there was significant impact. Peak flows declined by as much as 75 percent. The river channel filled in. As a result, when storms occur, the river is more likely to flood.
To help hold down soils, land managers introduced a new plant in the 1920s, native to Asia – the tamarisk, or salt cedar. It reduced sediment accumulation in the reservoir. But that wasn't its only effect.
Before tamarisk, cottonwoods and willows dominated the riverside habitat. And they were adapted to the cycle of spring floods. Not so for tamarisk.
“All summer long it's just pumping out seeds constantly – whereas the cottonwood only releases seeds to coincide with when the snowmelt ought to be coming downriver,” Cadol said of tamarisk. “So the fact that these floods happen and clear a little bit of land, put down nice silt that's a great bed for these seeds to grow in, the seedlings that start to grow are the tamarisk seeds, because the floods happen when there are no cottonwood seeds around.”
Tamarisk spread along the Rio Grande. Here's what happened in the Presidio Valley in Texas.
Cotton farming there expanded after the New Mexico dam was built. But when cotton prices dropped, this farmland was taken out of production. What happened next gave tamarisk a foothold. In 1942, there was a “spillover” at Elephant Butte. The flood washed silt onto the fallow fields. As a result, monocultures of tamarisk grew, from Candelaria to the confluence of the Rio Conchos.
Wildlife has been impacted. Deer and other herbivores once fed on grasses along the river, but thickets of tamarisk edged the grasses out. Meanwhile, other species adapted – like the endangered willow flycatcher, which now makes its nests in tamarisks.
Elephant Butte was a forerunner. In the decades that followed, dams were built on rivers across the West. These dams were usually built for a 50-year lifespan. Now, at 100 years, Elephant Butte has delivered on some of its promises. It's allowed the United States to meet water obligations to Mexico, and it's supported irrigated farming.
But it's also inefficient. Thirty percent of the water that reaches the reservoir is lost to evaporation. With the effects of climate change, snowmelt is expected to occur earlier. And this means evaporation losses will increase. Plus, the reservoir continues to fill with sediment.
Cadol wonders about the future of dams like Elephant Butte.
“It's definitely a critical question that's facing the water-management community, especially in the Southwest,” he said. “Hopefully there will be strong leadership that can guide us through that.”
One thing is clear – at a hundred years, Elephant Butte is a reminder that when we change nature, the results are always more complicated than we expect.