Despite the pervasive ecological impacts of river regulation, further regulation, in the form of weirs, levees and pumping, is being used to artificially inundate floodplains of the River Murray with an aim to improve floodplain health. In the lower River Murray, a 3 m high, 79 m wide regulator on Chowilla Creek has been constructed with the primary objective of maintaining or improving the condition of floodplain overstorey vegetation. The Chowilla Anabranch system comprises permanent lotic habitats that are now rare in the main channel of the lower Murray, and supports a regionally significant Murray cod (Maccullochella peelii) population. Operation of the Chowilla Regulator may alter the hydrodynamics of lotic habitats, interrupt connectivity, and decouple riverine and floodplain hydrographs. This presents risks for native fish whilst potentially facilitating recruitment of non-native fishes.
During spring 2014, the Chowilla regulator was used to inundate ~3,000 hectares of floodplain during relatively low river flows (≤10,000 ML/d). We quantified the hydraulic characteristics of perennial creeks, investigated the movement of Murray cod using radio-telemetry, and compared fish assemblages in ephemeral floodplain habitats between engineered and natural overbank flooding (discharge = 90,000ML/d). During regulator operation, mean water velocities and hydraulic complexity (strength and frequency of water circulation), were ~50% of those measured when the regulator was not in place. The regulator also obstructed the spawning movements of Murray cod. In floodplain habitats, native fishes were rare and young-of-the-year carp comprised 97% of the catch. During natural flooding, native fishes were numerically abundant, and carp represented 1% of the catch. Our results support the notion that engineered floodplain inundation simplifies and fragments aquatic habitats, and may benefit carp over native fishes. River regulation to promote ecosystem health represents an unparalleled experiment in lowland river restoration, requiring rigorous evaluation to provide feedback loops for adaptive management.