Reservoir cascades as directional invasion corridors

Abstract

Reservoir cascades in large river systems impose directional pathways that can structure biological invasions, yet how these pathways interact with propagule supply and lock-mediated passage opportunity to influence arrival timing remains poorly understood. We used interval- and right-censored time-to-event models to evaluate spread dynamics of silver carp ( Hypophthalmichthys molitrix ) across the Tennessee River and Cumberland River reservoir cascades and test whether the timing of first detection was explained by position within the cascade, annual water locked, or accumulated propagule exposure from downstream source populations. The timing of first detection was related to distance from the invasion source, but cumulative exposure metrics offered no supplementary model support, suggesting that arrival timing was mostly associated with position within the cascade. Models incorporating annual water locked offered modest supplementary support, suggesting that lock-mediated passage opportunity may partially influence the progression of the invasion front. Spread progressed directionally but was not complete in the Tennessee River cascade, indicating that pathway structure alone does not guarantee arrival. Patterns were consistent across both rivers, suggesting that spread through reservoir cascades can follow repeatable patterns in which arrivals are constrained by downstream source populations and lock-mediated connectivity. From a management perspective, our results suggest distinct leverage points across spatial and temporal scales, including early detection and containment at downstream reservoirs, suppression of source populations to slow spread, and interventions at invasion fronts or key connectivity points to limit further expansion. Recognizing reservoir cascades as directional dispersal pathways provides a simple and transferable basis for anticipating and managing invasions in large river networks.