Ephemeral streams, often overlooked, are major contributors to US river flow and water quality

Ephemeral streams – temporary streams that only flow after rainfall or snowmelt – contribute more than 50% of the flow in downstream river systems and likely have a major influence on water quality across the United States, according to a new modeling study. The findings show how important ephemeral streams are for the transport of water and pollution into larger, more permanent water bodies. Excluding these streams from coverage under the U.S. Clean Water Act, say the authors, would significantly limit federal authority to protect downstream water quality. Ephemeral streams, which flow only in direct response to precipitation and are disconnected from groundwater sources, play a crucial role in transporting nutrients, sediments, pollutants, and other materials to larger water bodies. Although these short-lived streams likely account for much of the global river network, research focused specifically on ephemeral streams is limited, and their hydrological contributions to downstream flow and water quality remain largely unknown. Craig Brinkerhoff and colleagues developed a model to quantify ephemeral stream contributions to more than 20.7 million more permanent water bodies in the contiguous US. Brinkeroff et al. combined data from published hydrological datasets to estimate ephemeral stream locations, and when and how much they flow. The authors found that ephemeral streams in the southwest and western US flow less frequently (only 4 to 46 days per year on average) compared to those in the eastern US, where they flow 173 days per year on average. However, despite their infrequent flow, western ephemeral streams contributed more significantly to downstream river flow – as high as 79% on average – than eastern ephemeral streams, which contributed ~50% on average. Combined, the findings show that ephemeral streams contribute, on average, 55% of the flow to the perennially flowing rivers in the contiguous US. According to Brinkeroff et al., the findings show that ephemeral streams are likely a substantial pathway through which pollution enters rivers, lakes, reservoirs, and ultimately the ocean. “Even though the ephemeral channels are often overlooked because of their infrequent flow, they are critical to downstream water availability,” write Judson Harvey and Stephanie Kampf in a related Perspective. “Climate- and land use-driven changes will alter flows and related functions of ephemeral streams in ways that influence future outcomes for water supply, drinking water quality, and the health of aquatic ecosystems in streams and rivers of all sizes.”