Satellite observations reveal widespread alteration of river thermal regimes by U.S. dams

Dams are well known to alter river thermal regimes, but assessments of downstream temperature changes have been constrained to single dams, single basins, or specific seasonal windows, thus limiting knowledge of their widespread impacts. We used satellite-based thermal infrared observations to quantify river surface temperature differences up- and downstream of 287 large dams in the United States from 2013 to 2024 across all seasons. We found downstream river temperature differences for the majority (71%) of longitudinal river profiles. These downstream changes were typically warmer (60%), and, on average, were sustained or continued to increase within 20 km downstream of the dam. We also found that the magnitude and frequency of downstream alterations varied by dam type, with 91% of extreme (≥ ±4°C) differences occurring at dams with reservoirs. This work documents the pervasive effects of large dams on downstream ecosystems across all seasons on a national scale.