From natural variability to flow homogenisation: how dams, water diversions, and climate change reduced seasonal flows in Australia’s Murrumbidgee River

River regulation and climate change have profoundly altered seasonal flow dynamics globally, with cascading ecological impacts on freshwater ecosystems and biodiversity. Magnitude and timing are key components of the flow regime, connecting rivers with floodplains and driving feeding and breeding cues for aquatic organisms. We investigated the separate and combined effects of water resource development and projected climate change on seasonal flow regimes in the lower Murrumbidgee River (~1500 km), a major river system within Australia’s Murray-Darling Basin. Using long-term hydrological data and rainfall-runoff modelling (1890-2022), we quantified changes in natural river discharge across key periods of river regulation and environmental flow management. Hydrological alterations, driven by dam constructions and water withdrawals, caused substantial homogenisation of seasonal flow patterns, reducing median river discharge by 24-68% between 1958-2022, compared to modelled natural flows. These declines persisted and intensified despite environmental water deliveries post-2006. Median climate projections (2047-2075) under CMIP6 SSP2-4.5 and SSP5-8.5 scenarios indicated further reductions in flows during the wetter seasons of autumn, winter, and spring (4-22%), exacerbating the impacts of river regulation, while summer flows may experience slight increases or decreases. These flow reductions have driven extensive drying of the semi-arid Lowbidgee Floodplain, a nationally important 3250 km2 wetland ecosystem, dependent on high interannual and seasonal flow variability for its “boom-and-bust” ecological processes. Flow homogenisation has disrupted natural flooding regimes, reducing floodplain connectivity, degrading riparian habitats, and facilitated the spread of invasive species at the expense of native biodiversity. Our research highlights the need for environmental flow management to better mimic natural flow regimes, prioritising large flow events to restore floodplain wetland health.