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Ephemeral streams have an overlooked impact on water quality

In the symphony of nature, ephemeral streams might seem like obscure players. However, these unassuming waterways, which flow only briefly after precipitation events, pack a punch.

Notably, they play a crucial role in the transfer of water – and all that it carries – from land surfaces to larger bodies of water.

A recent study has revealed the surprising extent of the importance of ephemeral streams, particularly regarding U.S. water quality.

The lasting impact of ephemeral streams

Though they might be out of sight, ephemeral streams leave a lasting impact on the nation’s waterscape.

Essentially, these temporary watercourses serve as transportation channels. They carry water pollutants, sediments, and nutrients from land to rivers, lakes, reservoirs and even to the oceans.

The reach of these streams is unexpectedly vast. According to the researchers, they influence a considerable portion of the water output from U.S. rivers.

The importance of these streams is in sharp contrast with their legal status in the United States.

Following a 2023 Supreme Court decision, these streams no longer fall under the jurisdiction of the Clean Water Act (CWA). This ruling, brought forth in Sackett v EPA, redefined the scope of the Clean Water Act.

The “waters of the United States” (WOTUS) now only encapsulate “relatively permanent, standing, or continuously flowing bodies of water” – a definition that conveniently sidesteps ephemeral streams.

Shedding light on the invisible streams

The research was conducted by scientists from the Yale School of the Environment and the University of Massachusetts-Amherst. The study was led by Craig Brinkerhoff, an incoming Yale postdoctoral fellow.

The team modeled ephemeral stream contributions to a colossal U.S. network of more than 20 million rivers, lakes, reservoirs, canals, and ditches.

The researchers found that ephemeral streams accounted for over 50 percent of the water in some of the nation’s largest rivers, like the Mississippi and Columbia. In some instances, the water contribution of ephemeral streams reached up to a staggering 90 percent.

Regardless of river basin size, ephemeral streams made up an average of 59 percent of drainage networks by length. Through these streams, pollutants such as nitrogen and pesticides are channeled into rivers, mirroring the streams’ water input magnitude.

“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,” noted the researchers.

Critics of the Supreme Court’s decision argue that it overlooked scientific findings. Instead, the decision hinged on dictionary definitions, resulting in a ruling that lifted protection on more than half of the annual discharge from U.S. drainage networks under the Clean Water Act.

“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,” said study co-author Doug Kysar, a professor at Yale Law School.

“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.”

By documenting the significance of ephemeral stream flow to downstream water quality, the results provide a basis for Congress to amend the Clean Water Act to expressly include ephemeral streams as an exercise of its power over interstate commerce, noted Professor Kysar. He said that the findings also point to the need 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,” said study co-author Peter Raymond. “These streams are a critical source of water and pollutants and have to be regulated.” 

All things are connected

Despite their unassuming nature, ephemeral streams have an influence that extends far beyond their immediate surroundings.

Brinkerhoff said the research 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.”

The study is published in the journal Science.


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