River regulation and climate change reduce river flows to major Australian floodplain wetland.

Freshwater ecosystems, including rivers and floodplain wetlands, face severe stress from unsustainable water resources development, with climate change exerting further pressure. This study compares the relative effects of river regulation and projected climate change on river flows to the semi-arid Lowbidgee Floodplain (3250 km), the largest wetland ecosystem on the heavily regulated Murrumbidgee River, Australia's second longest river, within the Murray-Darling Basin. We modelled annual natural streamflow in the lower Murrumbidgee River before major dam constructions and water diversions (1890-1927), linking river flows to runoff from the upper Murrumbidgee catchment. Extending this analysis to the full rainfall-runoff dataset (1890-2018), we compared modelled natural flows to observed river flows affected by dams and water withdrawals. Additionally, we modelled climate change impacts on river discharge and overbank flows, which reduced inundation of riparian habitats. Current river regulation has reduced median annual streamflow by 43% from 2565 × 10 m³ to 1490 × 10 m³ during 1958-2018, relative to modelled natural flows, with a more pronounced 55% reduction in the last three decades (1988-2018). The return period of major overbank flows, essential for river-floodplain habitat connectivity, more than doubled from once every 2.0 years to once every 4.4 years (1916-2018). Mean climate change projections indicated an additional 7-10% decrease in median annual streamflow by 2047-2075, relative to 1977-2005. The annual duration of major floods declined from an average of 11.3 days under natural flow conditions to 4.5 days under the current regulated river flow regime, with a further reduction to 1.6-1.8 days (83-85% decrease) projected by 2047-2075, due to climate change. We recommend prioritising mitigation of river regulation effects, as these pose the most immediate threats to riverine ecosystems, including their native biodiversity, in the Murrumbidgee River catchment. Our 'natural flow' model offers critical insights for shaping environmental policy and managing environmental flows to mimic natural flow regimes, supporting the conservation and restoration of freshwater ecosystems, like the Lowbidgee Floodplain wetlands. Our approach is transferable to other large river systems globally, using available or modelled streamflow data.