Evaluation of Floodplain Connectivity and Larval Fish Transport During an Extreme Event

Over the past two decades, the Missouri River has experienced several large-magnitude floods at a higher frequency than in previous decades. In 2019, flooding was persistent through the spring and summer with several record or near-record crests. Such events have the potential to increase floodplain connectivity in both batture areas as well as areas protected by levees that are breached during the flood. As the Missouri River is a highly-managed system with a constrained channel, it is important to consider the ecological implications of these floods, particularly if they occur at increasing intensity and frequency. We examine the possible effects of the 2019 event on the pallid sturgeon, an endangered fish native to the Missouri River basin, in the context of a concerted data-collection and modeling effort during and following the 2019 flood on the Missouri River at a site of ecological interest for the pallid sturgeon. After spawning and a period of incubation, larval pallid sturgeon drift passively to semi-passively downstream for a period of 9-14 days before transitioning from endogenous to exogenous feeding. This critical life-stage transition has been identified as a potential recruitment bottleneck. A bend of the Missouri River where large numbers of age-0 sturgeon have been captured is thought to be a location where this life-stage transition may be successful, and is a focus of ongoing studies to better understand drift and dispersal of pallid sturgeon. During flooding in 2019, we mapped velocity and topography of the bend over a range of discharges using acoustic Doppler current profilers (ADCPs) and a multibeam sonar. We documented several locations where both natural and constructed levees were breached, causing lateral exchange with the floodplain. Floodwater exchange has implications not only for alterations to drift hydraulics but also for productivity of sturgeon prey. Some flow occurred overbank beyond breached levees; preliminary analyses of ADCP data indicates approximately 3% of total discharge returned to the channel at one levee breach. We further evaluate this lateral exchange using a nested 1d/2d hydraulic model with particle tracking, and make inferences to other bends on the river through a survey of aerial imagery.