Ephemeral Streams’ Impact on U.S. Water Quality Revealed in Groundbreaking Study

A groundbreaking study reveals that ephemeral streams, recently excluded from Clean Water Act protections, play a critical role in the water quality of major U.S. rivers. The findings could spark legislative revisions.

Ephemeral streams, or those that flow only briefly after precipitation events, are a significant pathway for water transfer and have substantial implications for water quality, a recent study published in the journal Science has found. These streams, which transport pollutants, sediments and nutrients from land surfaces to larger bodies of water, contribute a remarkable proportion of the water output in the nation’s rivers.

However, in the wake of a 2023 U.S. Supreme Court decision, these vital waterways are no longer governed by the Clean Water Act (CWA). In Sackett v. EPA, the Supreme Court restricted the definition of “waters of the United States” (WOTUS) under the CWA to cover “only those relatively permanent, standing, or continuously flowing bodies of water.” This decision effectively excluded ephemeral streams from federal oversight.

The research team, led by Craig Brinkerhoff, an incoming Yale postdoctoral fellow, along with Peter Raymond, the Oastler Professor of Biogeochemistry at Yale School of the Environment (YSE), Matthew Kotchen, a YSE economics professor, and Doug Kysar, the Joseph M. Field ’55 Professor of Law at Yale Law School, modeled the contributions of ephemeral streams to over 20 million water bodies, including rivers, lakes and reservoirs, across the U.S.

“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 stated in their study.

The study revealed that more than 50% of the water in major rivers such as the Mississippi and Columbia originates from ephemeral streams at average annual discharge rates. This figure rises dramatically in some water systems, with the Rio Grande seeing over 90% of its water from these streams. On average, ephemeral streams account for 59% of U.S. drainage networks by length, carrying significant amounts of nitrogen, pesticides and other pollutants downstream.

“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 in a news release.

By emphasizing the crucial role of ephemeral streams, the study provides compelling evidence for Congress to amend the CWA to reinstate protective measures.

“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,” added Kysar.

He also pointed to the necessity for 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,” Raymond said in the news release. “These streams are a critical source of water and pollutants and have to be regulated.”

Brinkerhoff, who led the research as a doctoral student at the University of Massachusetts Amherst, noted the broader implications of the findings, suggesting that previously overlooked water systems have extensive impact.

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

In light of these revelations, the study calls for a re-evaluation of the current regulatory framework to ensure the protection and sustainable management of vital water resources.