Evaluating flow management as a strategy to recover an endangered sturgeon species in the Upper Missouri River, USA

Abstract

In the Upper Missouri River, Fort Peck and Garrison Dams limit the length of free‐flowing river available to the endangered pallid sturgeon. These barriers restrict the upstream migration of adults and downstream larval dispersal. A one‐dimensional (1D) modelling framework is currently in use to evaluate reservoir operation alternatives and to simulate drift of dispersing free embryos for different flow regimes and reservoir stages. This paper presents the results of a large‐scale tracer experiment conducted in 2016 and associated modelling performed to evaluate flow management scenarios that might aid species recovery. Breakthrough curves from the tracer experiment were used to infer the 1D longitudinal dispersion coefficient from a parameter optimization procedure. Simulations generated using the calibrated 1D advection–dispersion model were compared with field observations of the passive tracer and with larval fish collected during a previous experiment in 2007. When used with the appropriate range of dispersion coefficients, the 1D modelling framework agrees well with the available direct measurements of larval drift distances. Although we cannot unequivocally state whether insufficient length of free‐flowing river alone is causing recruitment failure, given the current thermal regime and our understanding of pallid sturgeon development, the time required for pallid sturgeon to transition to the benthos and initiate feeding might exceed the duration of drift available given constraints of reservoir operations.