Ephemeral streams likely to have significant effect on U.S. water quality

Ephemeral streams, or those streams that flow only briefly after precipitation events, are a substantial pathway for water transfer with significant implications for water quality, a first-of-its kind study has found. 

These streams — which transport water pollutants, sediments, and nutrients from land surfaces to rivers, lakes, reservoirs, and ultimately the oceans — influence a substantial amount of water output of the nation’s rivers, the researchers found. Following a 2023 U.S. Supreme Court decision, however, they are no longer regulated by the Clean Water Act (CWA). 

“Our findings show that ephemeral streams are likely a substantial pathway through which pollution may influence downstream water quality, a finding that can inform evaluation of the consequences of limiting U.S. federal jurisdiction over ephemeral streams under the CWA,” the researchers from Yale and the University of Massachusetts-Amherst said in the study published in Science.

In Sackett v EPA, the U.S. Supreme Court narrowly defined “waters of the United States” (WOTUS), within the scope of the CWA, as encompassing “only those relatively permanent, standing, or continuously flowing bodies of water” — effectively removing ephemeral streams from U.S. federal jurisdiction. 

For the new study, led by Craig Brinkerhoff, an incoming Yale postdoctoral fellow, and co-authored by Peter Raymond, the Oastler Professor of Biogeochemistry at Yale School of the Environment (YSE), Matthew Kotchen, professor of economics at YSE, and Doug Kysar, the Joseph M. Field ’55 Professor of Law at Yale Law School, the researchers modeled ephemeral stream contributions to the U.S. network of more than 20 million rivers, lakes, reservoirs, canals, and ditches.

In the nation’s largest rivers, such as the Mississippi and Columbia, more than 50% of the water originates from ephemeral streams at average annual discharge, they found. In some waterways, such as the Rio Grande, more than 90% of water comes from ephemeral streams. While the size of the river basin influences results, ephemeral streams on average account for 59% of drainage networks by length. These streams, the researchers say, pick up nitrogen and pesticides and other pollutants that are likely relayed to the rivers at similar magnitudes as their water input.

“When the Supreme Court narrowed the scope of the federal Clean Water Act, it did so by referring to abstract dictionary definitions rather than science. This research underscores the impact of that approach since, by our estimate, over half of annual discharge from U.S. drainage networks will no longer be protected by the Act,” Kysar said. 

By documenting the significance of ephemeral stream flow to downstream water quality, the results provide a basis for Congress to amend the CWA to expressly include ephemeral streams as an exercise of its power over interstate commerce, Kysar said, adding that findings also point to the need of enhanced regulation by state and local governments

“The chemistry of water is dependent on how you manage the entire watershed, not just pieces of it,” said Raymond. “These streams are a critical source of water and pollutants and have to be regulated.” 

Craig Brinkerhoff, who led the research while completing his doctoral degree at the University of Massachusetts Amherst, said it shows the vast impacts of waterways that were once considered to influence only their immediate areas.

“Our study provides more concrete evidence that all of these things are connected,” he said.