Federally unprotected streams contribute most of the water to US rivers

Previous research has shown that ephemeral flows play an important role in transporting sediments, chemicals and other materials downstream in larger bodies of water. So the new findings could shape future research and regulations targeting water pollution, says Jud Harvey, a hydrologist with the US Geological Survey based in Reston, Va.

“This study is the first to my knowledge that estimates how much [ephemeral streams] flow and how much of that flow is carried downstream,” says Harvey. “For many of us, it was a little surprising how much of the water we see in streams and rivers that are constantly flowing has come from these channels that are usually dry.”

The researchers used high-resolution maps of more than 20 million American rivers to track ephemeral flows across the country. Since these streams flow only with rain, their beds must lie above the groundwater level. Hydrologist Craig Brinkerhoff of the Yale School of the Environment and colleagues compared flow depths to average water tables using a previously published groundwater simulation.

The team then used another computer simulation to estimate the volume of water coming from these weather-dependent channels. By following the flow of water through smaller streams into larger regional rivers, “we kept track of those side contributions that came specifically from these ephemeral flows,” says Brinkerhoff. After the contributions were recorded, the researchers calculated the fraction of water flowing into the larger rivers that originally came from ephemeral streams.

Although the study is “pushing the envelope” to elucidate these understudied channels, its identification of transient flows may not be entirely accurate, says Ken Fritz, an ecologist at the U.S. Environmental Protection Agency, who is with center in Cincinnati. Computer simulation of groundwater cannot resolve differences smaller than a kilometer, says Fritz. The simulation also wasn’t meant to measure groundwater level fluctuations, so the researchers could have identified some streams as ephemeral that would also fit the definition for disruption. Unlike ephemeral streams, intermittent streams are fed by groundwater at least during some parts of the year (SN: 6/9/23).

One in three people in the United States get their drinking water from rivers and streams that the Clean Water Act seeks to protect from pollution. Knowing that more than half of that water comes from unprotected ephemeral streams “appears to have very significant implications for the ability of the United States — at the federal level — to ensure continued protection of clean water,” the Yale economist says. , Matthew Kotchen. , who co-authored the study.

The team hopes the work can serve as a scientific basis for creating better policies for rivers and water quality in the United States. “Scientifically, not regulating them doesn’t make sense if you want to reduce pollutants in your waterways,” says Yale ecologist and study co-author Peter Raymond. “You really want your policy to be based on science, and currently it’s not.”