Environment SA News

The flood of a generation

Floodwaters reach the Coorong

The flood of a generation

Record levels and flows

The Coorong, Lower Lakes and Murray Mouth (CLLMM) region continues to receive water from the largest South Australian flood in more than 50 years.

The influx of freshwater to the Coorong throughout the summer months led to some of the largest peaks in average daily water levels on record, at both the Southern and Northern lagoons.

It is only the second time since the barrages were constructed in the 1930s, that flows have been large enough to cause levels to rise significantly above normal.

Floodwaters peaked in the Lower Lakes approximately one month later and it is only the second time since the barrages were constructed in the 1930s, that flows have been large enough to cause lower lake levels to rise significantly above normal.

The prolonged high flow and barrage releases have led to significant and continued scouring of sand at the Murray Mouth, further deepening and widening the Mouth and improving the exchange of water between the ocean, the Murray estuary and Coorong.

While full water recovery via the implementation of the Basin Plan is important for the long-term ecological resilience of the Coorong, the 2022-23 flood event presents an exciting and unique opportunity to assess how the Coorong responds to this ‘once in a generation’ occurrence.

Water Quality

With the influx of fresh water, salinity has plummeted, resulting in some of the lowest average salinity readings on record.

Typically, the Coorong experiences its lowest annual water levels during summer or early autumn, corresponding with peaks in salinity around autumn. This year however, average salinity in the Southern Lagoon for January was recorded at 43 g/L, a significant decline from the 2022 seasonal peak (114 g/L).

Additionally, levels of nutrients, Chlorophyll-a and TRIX (a measure of trophic status, or productivity of the system) throughout November and December 2022 were all recorded lower than when compared to pre-2022 data.

With floodwaters aiding connectivity between lagoons and the flushing of excess nutrients and algae into the Southern Ocean, it has reiterated the importance of strengthening flushing and connectivity as a pathway to restoring the nutrient cycle of the Southern Coorong.

The continual collection of data throughout and after the flood event has provided valuable data to help understand how the system responds to high water levels and lower salinity. To observe Coorong water quality data in near-real time, please visit Water Data SA at Data - Water Data SA.

Ecological impacts

Whilst still early days, the positive ecological impacts of floodwaters on the Coorong are beginning to come to light.

Vegetation communities throughout the Lower Lakes, such as lignum and samphire, continue to show positive signs from an influx of water into wetlands.

Recent observations of sediments have indicated a marked increase in the presence of macroinvertebrates, which are known to remove nutrients. This is beneficial as sediments play an important role in the nutrient cycle of the Northern and Southern lagoons, and until recently sediment quality had been degraded, fueling eutrophication (high nutrient and organic matter loads).

Similarly, aquatic plants such as Ruppia have been seen in greater abundance. These plants pump oxygen from the water to their roots, which is then released into the sediment. This plays an important role in sediment-water nutrient cycling.

The ecological impacts of floodwaters are benefitting the Coorong food web with an increase in fish and waterbirds, particularly pelicans and swans, being observed.

Unfortunately, as some populations thrive, others have been negatively impacted. Shorebirds such as Fairy Terns have been observed in lower numbers due to their inability to forage on mud flats due to the increased water levels.

While the long-term effects of the 2022-23 flood event are yet to be seen, it is expected the greatest benefits for the Coorong food web and waterbird population will be observed in the next 1 to 2 years.