Federally unprotected streams contribute most of the water to U.S. rivers

The dry-looking stream in your backyard may play a major role in feeding U.S. rivers.

Channels that flow only in direct response to weather conditions like heavy rain, called ephemeral streams, on average contribute 55 percent of the water in regional river systems in the United States, researchers report in the June 28 Science.

But last year, the U.S. Supreme Court ruled that some waterways — including these streams — are not federally protected from pollution under the Clean Water Act. The decision could have a substantial ripple effect on the environment.

Previous research has shown that ephemeral streams play an important role in transporting sediment, chemicals and other materials downstream to larger bodies of water. So, the new findings may shape future research and regulations targeting water pollution, says Jud Harvey, a hydrologist at the U.S. Geological Survey based in Reston, Va.

“This study is the first to my knowledge that assesses how much [ephemeral streams] flow and how much of that flow is conveyed to downstream,” Harvey says. “For many of us, it was just a bit astounding how much of the water that we see in the perennially flowing streams and rivers has come from these channels that are typically dry.”

Researchers used high-resolution maps of more than 20 million U.S. rivers to track ephemeral streams across the country. Since these streams flow only with rain, their beds must lie above groundwater level. Hydrologist Craig Brinkerhoff of the Yale School of the Environment and colleagues compared stream depths against water table averages using a previously published groundwater simulation.

The team then used another computer simulation to assess the volume of water coming from these weather-dependent channels. Following the flow of water through smaller streams into larger regional rivers, “we kept track of those lateral contributions that specifically came from these ephemeral streams,” Brinkerhoff says. Once the contributions were logged, researchers calculated the fraction of the water flowing in the larger rivers that originally came from ephemeral streams.

Though the study is “pushing the envelope” to clarify these understudied channels, its identification of ephemeral streams may not be entirely precise, says Ken Fritz, an ecologist at the U.S. Environmental Protection Agency who is based in Cincinnati. The computer simulation of groundwater can’t resolve differences smaller than a kilometer, Fritz says. The simulation also wasn’t meant to measure groundwater level fluctuations, so the researchers might have identified some streams as ephemeral that would have also fit the definition for intermittent. Unlike ephemeral streams, intermittent streams are fed by groundwater during at least 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. The knowledge that more than half of that water comes from unprotected ephemeral streams is “likely to have very significant implications on the ability of the United States — at the federal level — to ensure the continued protection of clean water,” says Yale economist Matthew Kotchen, who coauthored the study.

The team hopes the work can serve as a scientific basis for creating better policies on rivers and water quality in the United States. “From a scientific point of view, not regulating them makes no sense if you want to decrease the pollutants in your waterways,” says Yale ecologist and study coauthor Peter Raymond. “You really want to have your policy grounded in science, and it’s currently not.”