The NSW Reconnecting River Country Program is exploring ways to relax physical, policy and operational constraints to the delivery of water for the environment to wetlands and low-lying floodplains in the Murray and Murrumbidgee catchments. While relaxing constraints has shown benefits for ecosystems of both the floodplain and adjacent river, there is a potential risk that water returning from the floodplain will contain high concentrations of dissolved organic carbon (DOC). During warmer months high DOC concentrations can accelerate rates of microbial metabolism, resulting in low concentrations of dissolved oxygen that can be fatal to aquatic fauna. Several recent large-scale hypoxic events have occurred in the Murray, Murrumbidgee and Edward-Wakool Rivers that were caused by natural floods during warm periods.
The purpose of the study was to assess the likelihood that flow regime changes proposed in the Program would increase the frequency of hypoxic blackwater events. We used a modelling approach to predict the occurrence of hypoxic blackwater from modelled hydrological regimes with different operational limits on the delivery of environmental water.
Our models predicted that hypoxic blackwater events are uncommon in both systems, with 6 events predicted in 124 years of model timeseries data for the Murray System and 11 events for the Murrumbidgee linked to large, natural flow events. These results suggest that, given the volume of environmental water available and operational guidelines, it is unlikely that planned environmental flows by themselves could result in large scale hypoxic events. Further, the risk can be assessed beforehand through modelling, and the planned flow changed to minimise the risk. The outcomes of this study provide insights into the role of environmental flows in creating and mitigating hypoxic blackwater and provide ways to evaluate hypoxic blackwater risk in the future.