The article examines the impact of dams on regional river dynamics and its implications for global biodiversity. The authors use long-term streamflow records from 186 intermediate-sized rivers across the continental United States to show that dams have homogenized flow regimes in 16 historically distinct hydrologic regions over the 20th century. This homogenization primarily affects the magnitude and timing of ecologically critical high and low flows, reducing regional differences that are crucial for maintaining biodiversity. The study finds no evidence of homogenization in undammed reference rivers, despite documented changes in regional precipitation. With an average density of one dam every 48 km of third- to seventh-order river channels, dams may be creating conditions that favor the spread of non-native species at the expense of native species. The findings highlight the need for conservation and management actions to restore and maintain native biodiversity and ecosystem function in response to the continental-scale effects of damming.The article examines the impact of dams on regional river dynamics and its implications for global biodiversity. The authors use long-term streamflow records from 186 intermediate-sized rivers across the continental United States to show that dams have homogenized flow regimes in 16 historically distinct hydrologic regions over the 20th century. This homogenization primarily affects the magnitude and timing of ecologically critical high and low flows, reducing regional differences that are crucial for maintaining biodiversity. The study finds no evidence of homogenization in undammed reference rivers, despite documented changes in regional precipitation. With an average density of one dam every 48 km of third- to seventh-order river channels, dams may be creating conditions that favor the spread of non-native species at the expense of native species. The findings highlight the need for conservation and management actions to restore and maintain native biodiversity and ecosystem function in response to the continental-scale effects of damming.