What waterways contribute to over 50 percent of downstream river systems but are no longer covered by the Clean Water Act? Dry for much of the year and only forming after weather events such as rainfall or snowmelt, ephemeral streams have only recently been studied by researchers from the University of Massachusetts Amherst modeled over 20 million rivers, lakes, and reservoirs to determine their pollution impact. 

Their discovery was astonishing: 55 percent of river system water discharged into oceans originated in ephemeral streams. That number increased to an average of 79 percent the western part of the United States with discharge from the Black Rock Desert, Nevada, and Humboldt County, California rising to 94 percent.

The Supreme Court excused ephemeral streams from the Clean Water Act in 2023’s Sackett v. Environmental Protection Agency by ruling that protected water bodies would include “only those relatively permanent, standing or continuously flowing bodies of water forming geographical features that are described in ordinary parlance as streams, oceans, rivers, and lakes.” This ruling left the majority of U.S. river ways susceptible to unregulated pollution brought to them through epemeral streams.