Oral Presentation World Lake Conference 2025

Floodplain Lagoons and Groundwater Interactions Under the 2022–23 River Murray Flood (#83)

Yuyang Zhu 1 , Chenming Zhang 1 , Yuanchi Xiao 1 , Juliette Woods 2 , Jess Thompson 3 , Harald Hofmann 1 4
  1. University Of Queensland, St Lucia, QLD, Australia
  2. Department for Environment and Water, Adelaide, South Australia, Australia
  3. Murray-Darling Basin Authority, Canberra, ACT, Australia
  4. Commonwealth Scientific and Industrial Research Organisation (CSIRO), Brisbane, QLD, Australia

The Lower River Murray region in South Australia features shallow, saline aquifers that interacts with fresh river water and influence the adjacent floodplain environment. Freshwater lagoons formed by flooding and environmental watering within these floodplains provide crucial water resources for local ecosystems and serve as key components in managing and reducing floodplain salinity. In late 2022, the River Murray experienced its highest flood since 1956, inundating extensive floodplain areas, altering local hydrological conditions and creating new wetlands, billabongs and lagoons. This event provided an opportunity to observe Freshwater lagoons and their interactions with saline aquifer in a semi-arid floodplain.

We conducted continuous surface water and groundwater monitoring at Murtho, South Australia, capturing the full life cycle of a temporary lagoon. In situ moisture monitoring and geophysical surveys (Electrical Resistivity Tomography) were applied to offer insights into soil water distribution and salinity changes. The results demonstrate that:

  1. Flood inundation resulted in the formation of multiple lagoons on the floodplain disconnected from the main river, with inflow subsequently limited to rainfall events. These fresh shallow surface water bodies remained for durations of approximately 6–10 months. Throughout the observed lagoon lifecycle, water loss was evenly divided, with approximately half lost to evaporation and the remainder infiltrating into underlying soils.
  2. Lagoon water preferentially infiltrated through desiccation cracks and vegetation root zones within the overlying low-permeability clay, initially stored as soil water. Subsequent declines in river stage and associated hydraulic head reductions of aquifer facilitated the recharge of stored soil water into the underlying floodplain groundwater.
  3. After lagoon dried, fresh soil water in the upper 1.5 m of the clay layer temporarily sustained vegetation growth until depletion by evapotranspiration. Conversely, deeper soil moisture persisted longer, becoming increasingly saline due to interaction with salts previously accumulated within the clay.

In conclusion, while lagoon inundation temporarily supported vegetation recovery and reduced near-surface salinity, the floodplain remains vulnerable to salinity risks following lagoon desiccation. Continued monitoring is essential to track post-inundation dynamics and inform adaptive salinity management.