History, hydrology and hydraulics: Rethinking the ecological management of large rivers

Abstract Climatic extremes capture imaginations and provide a fundamental premise for biologists—that ecosystems are adapted to natural variability. Hence, understanding past extremes provides a template for contemporary ecological models and management. Nevertheless, myths can develop around historical climatic events, distorting perceptions of the past. The mythology of the Murray River in Australia is that over 100 years ago, it naturally “dried to a series of pools” in drought; therefore, the biota are flexible and adapted to hydrological variability and lentic habitats. Analysis of historical and modelled hydrology and hydrodynamics, however, demonstrates that: (a) cease‐to‐flow events were not natural and were instead caused by multiple small‐scale irrigation diversions; and (b) the Murray River had widespread perennial lotic habitats. Within a generation, the spatial, temporal, and causal context was lost and with it, the links between preregulation hydrology and hydraulics, and river ecology. From an intermittently lentic system, we propose an alternative model which integrates ecohydrology and ecohydraulics. Specifically, the model incorporates: (a) persistence of lotic in‐channel and lentic off‐channel refugia, even in droughts; and (b) a reliable spring flow pulse that increases hydrodynamic complexity, promotes longitudinal integrity of lotic conditions and replenishes low‐lying wetlands. The model helps explain the decline of lotic biota, suggesting that hydraulic change has had a greater impact on aquatic biodiversity than changes in hydrology. Being mindful of historical conditions and considering spatio‐temporal ecohydraulics provides new opportunities for the rehabilitation of highly modified rivers and may assist the strategic development of large rivers, including for hydropower.