Flood‑Induced Changes in Riverbed Salinity Observed Using Instream Hydrogeophysics in a Semi‑Arid River System

River regulation has contributed to increased soil and groundwater salinity across many floodplains in semi-arid Australia. Floodplain inundation has the potential to flush salt from the soil profile and reduce groundwater salinity in shallow aquifers. However, the effectiveness of flood events in removing accumulated floodplain salt remains poorly constrained. This uncertainty reflects the long-term legacy of salt accumulation, spatial variability in groundwater depth, and changes in the capillary transport zone. Salt movement within floodplains is influenced by multiple interacting factors, including river geomorphology, shallow aquifer properties, remnant paleochannels, and hydraulic gradients between surface water and groundwater. To assess the impact of flooding on salt redistribution at the river–floodplain aquifer interface, instream hydrogeophysics surveys (transient electromagnetic-TEM) were conducted along Katarapko Creek and the River Murray at Bookpurnong, South Australia, following the major 2022–2023 River Murray flood event. These surveys mapped spatial variations in riverbed electrical conductivity and identified potential zones of saline groundwater inflow. Comparisons with surveys undertaken in 2015 and 2019 reveal substantial post-flood changes in riverbed conductivity, including an overall reduction in conductivity. A follow-up survey in 2024 confirmed that the spatial distribution of conductivity features remained consistent across all survey periods. Despite the general decrease in riverbed conductivity following the flood, several discrete zones continue to act as preferential pathways for saline groundwater discharge from the floodplain to the river. The persistence of these salinity hotspots indicates that considerable salt stores remain within the floodplain system. These findings suggest that while large flood events can reduce near-surface salinity, targeted adjustments to river regulation may be required to restore key hydrological processes and support long-term ecological recovery of the river system.