Ephemeral stream water contributions to United States drainage networks.

Ephemeral streams flow only in direct response to precipitation and are ubiquitous landscape features. However, little is known about their influence on downstream rivers. Here, we modeled ephemeral stream water contributions to the contiguous United States network of more than 20 million rivers, lakes, and reservoirs, finding that ephemeral streams contribute, on average, 55% of the discharge exported from regional river systems, as defined by the United States Geological Survey. Our results show that ephemeral connectivity is a substantial pathway through which water and associated nutrients and pollution may enter the perennial drainage network and influence water quality. We provide quantitative insight into the implications of differing interpretations of regulatory jurisdiction under the United States Clean Water Act, including the current standard adopted by the Supreme Court of the United States in 2023.