Of the roughly 600,000 cubic feet (17,000 cubic meters) of water that spills each second from the Mississippi River into the Gulf of Mexico, half comes from streams that don’t even exist for part of the year.
So-called ephemeral streams, which sit above the water table, supply 51% of the Mississippi’s water. That’s on par with the average for U.S. rivers, according to a new study published in Science. This is one of the first times the role of ephemeral streams in contributing to downstream flows has been quantified, and the analysis comes at a pivotal moment for regulations around these kinds of less obvious waterways.
“By demonstrating how much of the water comes from [headwaters and ephemeral streams], it gives us a context for how important they will be to regulation of environmental quality.”
Following a 2023 U.S. Supreme Court decision, EPA and other federal bodies lost their authority to regulate some kinds of waters within the United States, including most ephemeral streams. Critics of the ruling say this could leave even still-regulated rivers and lakes vulnerable, as smaller bodies of water feed into them. But hard numbers on what those contributions are have often been lacking. This new study helps to change that.
“There are exponentially more headwaters and ephemeral streams than there are large rivers,” said Adam Ward, a hydrologist at Oregon State University who wasn’t affiliated with the research. “By demonstrating how much of the water comes from those, it gives us a context for how important they will be to regulation of environmental quality.”
Only Sometimes a Stream
Ephemeral streams sit above the water table, meaning they are fed only by precipitation. That means most sit dry for stretches of time—some in the arid southwestern United States flow only a few days each year, on average.
To find these on-again, off-again waterways, the study’s authors took an existing map of stream channels in the contiguous United States and overlaid it on a groundwater model. By excluding streams that dipped below the modeled average level of groundwater, they were able to pick out most ephemeral streams. Then they validated a portion of those streams with existing site assessment data to make sure their model was accurate.
Their data showed that 55% of the water coming from rivers in the contiguous United States begins in ephemeral streams, though large regional variations exist. The percentage of river flow derived from ephemeral streams is much higher in the West than the East, for example, and very low overall in the upper Midwest, where the water table is shallow.
The results underline the fact that ephemeral streams are an important factor in water flow across the country, said Craig Brinkerhoff, a hydrologist at Yale University and the study’s lead author.
“We usually think ephemeral streams are characteristic of the desert,” he said. But trace river headwaters back far enough in many places, and you’ll find seasonally dry gullies that become streams only when rain falls.
How Far Does the Clean Water Act Extend?
This new quantification of where the water in major waterways originates has implications for a long-standing debate about which waters federal agencies such as EPA and the Army Corps of Engineers (which issues permits for development work near bodies of water) can regulate.
The Clean Water Act, passed in 1972, is the primary federal law governing water quality in the United States. But which bodies of water the law covers has long been a subject of debate. Part of that confusion stems from a 2006 Supreme Court case that said it applied only to “relatively permanent, standing or flowing bodies of water” and wetlands with a “continuous surface connection” to them. A concurring opinion from Justice Anthony Kennedy left the door open to waters with a “significant nexus” to those covered waters, an imprecise definition that allowed federal agencies to extend protections more broadly.
In 2023, the Supreme Court revisited the matter in Sackett v. Environmental Protection Agency, this time removing the “significant nexus” language and narrowing the law’s scope to just relatively permanent bodies of water such as rivers and lakes, as well as those with a continuous surface connection to them. That means that ephemeral streams, which are not, by definition, “relatively permanent,” don’t count.
“All of the systems are connected.”
Brinkerhoff argued the new research shows we may need that more expansive definition, however. With half the water in rivers coming from ephemeral steams, significant amounts of pollution resulting from underregulation could come with it.
“All of the systems are connected,” he said. “If you turn regulation on and off, you can still end up with water in nominally regulated water bodies that might have come from an unregulated place.”
The research doesn’t alter the definition of an ephemeral stream, but by quantifying how much water these streams contribute to protected bodies, it does highlight their significance in a way that could cause lawmakers to take note, Ward said.
“They can see very clearly now [that] more than half of the water in the rivers they intended to protect originates in ephemeral streams and headwaters,” he said. “It seems a logical outcome of that would be to explicitly say, protect the ephemeral streams and headwaters.”
—Nathaniel Scharping (@nathanielscharp), Science Writer
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