Pennock, C.A, L. Bruckerhoff, K.B. Gido, A.L. Barkalow, M.J. Breen, P. Budy, W.W. Macfarlane, and D.L. Propst. 2022. Failure to achieve recommended environmental flows coincides with declining fish populations: Long-term trends in regulated and unregulated rivers. Freshwater Biology https://doi.org/10.1111/fwb.13966. USGS FSP IP- 134441 (LB).
Dams can be operated to mimic components of the natural flow regime to minimize impacts on downstream ecosystems. However, infrastructure, societal needs, and watershed runoff constrain which and when flow regime attributes can be mimicked. We compared fish assemblage responses, including native and nonnative species, over two decades of managed environmental flows to those in a river retaining a relatively natural flow regime. Both of these arid-land rivers are within the Colorado River basin and have experienced declines in watershed runoff over the past twenty years. We predicted fish-flow relationships would be conserved across time and between managed and unmanaged rivers. Declines in flow in both rivers coincided with declines in some native fishes, and more native and nonnative fish species exhibited declines in the managed river than in the unmanaged river. Our ability to detect previously documented native fish-flow relationships diminished in the managed river system because established environmental flow targets were not met due to water management, but we detected these fish-flow relationships in the unmanaged river. Our results suggest declining watershed runoff and increased consumptive water use could reduce the effectiveness of environmental flows that have lower priority in most years.