Science

Climate change is drying out many part-time streams in the United States

Climate change is altering intermittent streams, such as this one in California’s Death Valley.

TIM FITZHARRIS/MINDEN

Small streams that dry up for part of the year are easy to overlook. But these intermittent streams are everywhere, making up more than half of Earth’s waterways. They help purify surface water and provide crucial habitat for creatures such as the Sonoran Desert toad, fairy shrimp, and Wilson’s warbler. Now, a study has found that ephemeral streams across the continental United States have become less reliable over the past 40 years, likely as a result of climate change. Some are dry for 100 days longer per year than in the 1980s. “That’s really shocking,” says Sarah Null, a watershed scientist at Utah State University.

The findings, reported last month in Environmental Research Letters, come from a study of data collected between 1980 and 2017 by flow gauges on 540 intermittent streams around the United States. Most of the gauges were on small waterways in river headwaters, but a few tracked large rivers that are intermittent in places, such as the Rio Grande, which flows sporadically in New Mexico and Texas. The sample covered just a small fraction of intermittent streams, the authors note, and left out some states, such as Nebraska and Maine, that don’t have any long-term gauges on these streams. Still, the analysis revealed some eye-opening regional shifts, says Sam Zipper, one of the authors and an ecohydrologist with the Kansas Geological Survey.

More than half of the gauges showed changes in the streams’ flow patterns since 1980. Some now shrivel earlier in the year and remain dry for longer, for example, or they dwindle more quickly than before. At some 7% of gauges, dry periods expanded by 100 days or more.

That is bad news for the many plants and animals that time their reproduction to the availability of water, especially in deserts. “Just because the channel is dry does not make it biologically dead,” says river scientist Ellen Wohl of Colorado State University. It also has implications for water quality, as microbes in damp sediments can remove nitrogen pollution even after the last puddles have disappeared.

The drying trend is clearest in arid regions, such as the Southwest. But even in the Southeast, which is relatively wet, streams are drying earlier and staying dry longer. In contrast, in the northern United States ephemeral rivers are now flowing longer. One possible reason: Winters are warmer and shorter, meaning frozen landscapes thaw earlier, allowing streams to flow.

In some cases, human activities such as operating dams, irrigation, and groundwater pumping could be contributing to dewatering. But a warming climate appears to be “the overarching organizer” of the shifts, Zipper says. “I definitely didn’t expect the pattern to be so regionally clear.”

Broader monitoring of intermittent streams would help researchers and policymakers better understand the sometimes subtle impacts that climate change is having on water quantity and quality, scientists say. “We should have many more gauges in small streams,” says Albert Ruhi, a freshwater ecologist at the University of California, Berkeley.

Others say the results highlight the need for stronger legal protections for intermittent tributaries that form the headwaters of many rivers. Many such streams were excluded from federal environmental laws under former President Donald Trump’s administration. (President Joe Biden’s administration is now reviewing those exclusions.) Such streams can seem inconsequential, Wohl says, but, “If you start chopping off the first joint of each finger, you’re going to lose functionality in your hand pretty fast.”

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