A new map points out U.S. rivers most worth protecting

CHICAGO, 1 OCTOBER 2026 —

A team of U.S. scientists has released the first continent‑wide map that grades every river and stream in the lower 48 states, identifying which waterways are already resilient and which require a single targeted repair to restore ecological health, the researchers said on 16 September in the journal PLOS One.

The four‑year project evaluated one‑third of the United States’ stream distances, rating each small watershed on four criteria: connectivity of the river network, flood‑plain condition and water quality, water availability, and the degree to which flows have been altered. The scores were overlaid on 91 existing datasets that locate concentrations of freshwater species.

Dozens of scientists and partners from 44 states adjusted the metrics to reflect regional differences ranging from the humid East to the arid Great Plains and the deserts of the Southwest.

According to Mary Khoury, a freshwater ecologist with The Nature Conservancy in Chicago, the new map “allows us to point at a map for the first time and say, these are the rivers most capable of sustaining diversity of aquatic life, and this is what each one needs.” The healthiest rivers cluster along the Pacific Coast, the northern Rockies and the Southeast, while the most vulnerable stretches lie in the Southwest deserts, the Great Plains and southern New England.

More than half of the mapped stream distances scored high on all four criteria and were classified as “resilient.” The remaining waterways were deemed “restorable,” meaning they are still functional but are constrained by a single issue such as agricultural runoff or a dam that blocks fish passage. Together, the resilient and restorable categories constitute what the authors term a “freshwater resilient and connected network,” a priority set for future protection and restoration.

Dams emerged as the most significant source of fragmentation, breaking the nation’s rivers into more than 9,000 separate segments. The study pinpoints where reconnection would be most effective, citing the Klamath River in Oregon and California—where lower‑dam removal is the largest dam‑removal project in U.S. history—and the Delaware River, which flows unimpeded from New York’s Catskill Mountains to the Atlantic Ocean, as examples of long, unbroken stretches within the resilient network.

Rafael Schmitt, a hydrologist at the University of California, Santa Barbara who was not involved in the work, said the map could later be overlaid with dam‑license expiration dates to identify the most impactful restoration opportunities.

Unlike terrestrial ecosystems, where the healthiest areas tend to be under formal protection, the study found that river health is largely independent of protection status. Thirteen percent of the country’s stream distances are in good condition but lack safeguards, roughly double the 6 percent that are both healthy and protected.

The authors warn that unprotected healthy rivers are at heightened risk of degradation because upstream activities—such as farming, dam operation or urban development—can quickly erode water quality and habitat connectivity. “It matters what’s going on upstream of that protected area,” Khoury said.

For Malaysia and the broader Southeast Asian region, the findings underscore the importance of integrating water‑way health into conservation planning, especially as climate‑induced temperature rises and altered flow regimes threaten freshwater biodiversity. The U.S. map offers a template for how large‑scale, data‑driven assessments can guide investment toward the most cost‑effective interventions, a lesson that could inform regional river‑restoration initiatives under the ASEAN Water Security Framework.

The authors intend to update the map as new data become available and to explore how policy levers such as dam‑license renewals or protected‑area designations can be aligned with the identified priority sites. The study provides a concrete basis for directing limited conservation resources toward the rivers most capable of sustaining aquatic life in a warming world.

Related: The Nature Conservancy · Chicago