Executive Summary:The Coorong, Lower Lakes, and the Murray Mouth form a terminal lake system at the mouth of the River Murray in South Australia.A line of barrages inside the mouth separate the Lower Lakes (Alexandrina and Albert) from the saline waters of the Coorong which exchange with the sea through the Murray Mouth.The Coorong and the Lower Lakes are of national and international conservation status especially for birds and are listed as Ramsar Wetlands.Reduced river flows in the River Murray in recent years and the associated increased likelihood of Mouth closure are regarded as a threat to the ecological function ofthe Coorong through the propensity for higher salinities in the system, alterations to the water level regime, and blockage of fish migration pathways.Water level is a key environmental attribute that determines the availabiltiy of physical habitat for birds and for other aquatic life.Similarly, all aquatic organisms have tolerance limits to salinity levels and their degree of variation, so the salinity regime within the Coorong is a major determinant of where and how well such organisms can live and prosper.The physical environment of the Coorong can be altered through human manipulation including dredging the Mouth and by altering the timing and magnitude of barrage discharges and releases from the Upper South East Drainage scheme (USED) into the South Lagoon.This report considers the factors that determine water levels and salinity within the Coorong as they are.The main results can be summarised as:1.Water levels in the Coorong udnergo a seasonal cycel of up to ~0.7m in range, higher levels tending to cocur in late winter-early spring and lower in late summer-early autumn.This seasonal variation is due to a combination of variation in sea level outside the Mouth and back-up due to discharge through the barrages.2.Shorter term water level variations of ~± 0.05m typically are due to the tilting of the water suface by the wind.Tidal level variation is important near the Mouth.3.Salinities increase along the Coorong from the Mouth through to the South Lagoon where salinity reaches several times that of sea water. The sloshing of water in and out of the South Lagoon associated with seasonal water level variation is a key determinant of the salinity there.4.Salinity stratification has been observed to occur in the North Lagoon and has the potential for negative ecological consequences, but its causes are poorly understood.5.Variations in water level and salinity and the exchange of other material within the Coorong are inextricably linked.Manipulation of water levels through discharge variation or Mouth dredging will have consequences for the salinity regime and for the concentrations of substances such as nutrients.
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