Historical Changes in Streamflows Important to Diadromous Fish in United States Coastal Regions

ABSTRACT

Diadromous fish species occupy freshwater habitats (rearing, spawning, migration) for a substantial part of their lives. The quality and quantity of these habitats is mainly driven by thermal and streamflow regimes; therefore, understanding trends in streamflow allows better management of diadromous species and their habitats. We investigated seasonal flows for many basins in two U.S. regions on the Pacific coast and two on the Atlantic coast and computed seasonal trends for eight streamflow metrics for each region from 1973 to 2022 and from 1943 to 2022. In the Atlantic regions, February through May maximum streamflows and frequency and duration of high flows mostly decreased from 1973 to 2022. These trends toward drier conditions coincide with migration windows of multiple diadromous species. Summer low‐flow frequency and duration in the two Pacific regions increased (toward drier conditions) in many near‐natural basins. More low‐flow days can affect species directly through decreased river longitudinal connectivity, which can limit habitat access and food availability. Trends in highly regulated systems differed from trends in near‐natural systems, especially in the Pacific regions, because large reservoirs often decrease peak flows and increase low flows. For example, in the southwestern U.S. coastal region, low‐flow duration and frequency significantly decreased (wetter) in many regulated basins while near‐natural basins were not similarly affected. Our results demonstrate how analysis of environmental flows at regional and seasonal scales can inform future habitat management and restoration plans.