Major changes in estuarine water quality following flood events are influenced by the duration of floodwater-floodplain contact and the dynamics of floodwater recession as controlled by floodplain geomorphology. Episodic large-scale deoxygenation events causing mass mortality of benthic and aquatic biota can follow these events. This study examines the timing and sources of poor quality water entering a sub-tropical coastal estuary in eastern Australia following a flood event. The results have implications for targeted river and floodplain management to improve water quality.
The Richmond River is a mature barrier estuary with a large floodplain to upper catchment ratio. The lower floodplain is characterised by extensive, well-developed natural levees grading into large backswamp basins. Prior to European modification of floodplain hydrology, these backswamp basins were semi-permanent wetlands and natural storage basins for floodwaters occluded from the main channel by the morphology of natural levee systems.
Following a 1:10 year flood event, the Richmond River estuary was closed to commercial and recreational fishing due to a fish-kill event attributed to deoxygenation of >30km of the main channel. Floodwater samples were collected along the estuary for one month following the flood peak and aqueous geochemistry analysed. Upper catchment inflows to the estuary remained well-oxygenated during the event and therefore, were not the source of the deoxygenating water. The primary source of the deoxygenating waters during the flood recession phase were the major mid-estuary backswamp basins identified by their unique anaerobic geochemical signature. Large constructed drains, which alter floodplain hydrology, dissect the natural levees bordering backswamp basins to allow floodwaters to drain from the backswamps into the main river channel at an enhanced rate. Whilst lower quality floodwaters were previously largely retained in backswamp basins during the flood recession phase, these waters are now exported to the main river channel causing declines in river health.