As a result of the Supreme Court decision on the upstream reach of antipollution laws, half of the water in U.S. rivers comes from so-called ephemeral streams that are now without federal protection, said researchers from the University of Massachusetts and Yale on Thursday. Those streams, which flow in response to precipitation, “collectively convey on average 55 percent of the flow to perennially flowing rivers in the conterminous United States,” said the researchers in the journal Science.
“The analysis has implications for downstream water quality and water resources management in the United States and elsewhere,” they summarized. Craig Brinkerhoff of the University of Massachusetts, one of the researchers, said material transported by ephemeral streams could “ultimately influence water many kilometers away that is, at least nominally, still regulated by the Clean Water Act.”
Farming, mining, and construction groups said the ruling in Sackett v. EPA in May 2023 was a victory for property rights. The Supreme Court ruled that federal law protected wetlands only if they had “a continuous surface connection” to streams, oceans, rivers, or lakes, and that those protections did not extend to channels with intermittent flows of water. Environmental groups said the decision, which also narrowed federal protection of wetlands, would jeopardize drinking water supplies for millions of people.
River basins west of the Mississippi River, where there’s less annual precipitation, were more influenced by ephemeral streams than those in the eastern half of the country, said the researchers. Even so, “in a humid place, where there’s tons of groundwater, ephemeral streams are still exerting a big influence,” said Brinkerhoff.
For their work, the researchers built a model that identified every ephemeral stream in the 48 contiguous states and calculated each one’s contribution to average streamflows.