Case study: The River Murray
Under natural flows the River Murray was an important source of sediment to the coastal waters off its mouth. Harris (1995, figure 5) estimates that it would deliver on average 30 million tonnes of sediment per year to the sea (compare with the Burdekin, which averages 45 million), thus creating a significant area of near shelf surface sediments not primarily of marine carbonate materials, rarely found between Victoria and north-west Western Australia. This natural discharge has now been altered. Massive clearance and elaborate river regulation have altered flows of water and sediment across and within slopes and floodplains, from river banks and channel floors, so that sediment entrainment, movement and deposition are now quite different from the situation in the mid nineteenth century. Most notably in this context, the River Murray Barrages, completed in 1939, now separate the river and terminal lakes Albert and Alexandrina from the sea.
Shaffron et al. (1990) noted that turbidity varies with flow within the Murray-Darling Basin: turbidity in the Darling was 25 NTU in May 1983 during drought, while in the flood of the following month this rose to 500 NTU. However, average turbidity generally increases downstream (figure 3.2).
Figure 3.2 River Murray turbidity (median values 1978-88)
Source: Shaffron et al. 1990, p. 152
While the concentration of suspended sediment increases downstream, water discharge decreases rapidly. This was the case under natural flow, as the waters from the eastern highlands were reduced by the high evaporation conditions of western New South Wales and South Australia. This loss of flow is greatly increased by diversions for agriculture: over half the natural flow is taken up in this way. The extent to which this reduces sediment discharge is uncertain. The greatest sediment transport could be assumed to occur at times of flood. The very largest floods (every 10-20 years) are allowed to flow through the system. However, small to medium size floods, which might occur once or twice a year, have been almost eliminated through river regulation.
Within the Murray-Darling Basin, urban sewage treatment plants are more important sources of nutrients than irrigation drainage or urban stormwater systems. Creagh (1992) noted that 500 tonnes of phosphorus is added to the river annually from treated sewage, irrigation drainage adds 260 tonnes in a wet year and 120 in a dry year, and urban stormwater adds 130 tonnes in a wet year and 60 in dry years. Comparable figures were given for nitrogen: urban sewage treatment plants adds 280 tonnes to the river. While it is clear that nutrient levels in the river are much higher than under natural circumstances, the significance of this to the estuarine and marine areas of the Murray Mouth and the Coorong lagoon is uncertain, except in as far as releases from the barrages may often contain algal blooms from the high-nutrient terminal lakes.
Today most sediment from the Murray-Darling Basin is accumulating in the terminal lakes, as well as in the storages constructed throughout the drainage basin. Discharge past the barrages is managed to release large flows and at other times to reduce shoaling at the Murray Mouth. Outflow to the Murray Mouth is now only four fifths of natural flow (Jensen et. al, 2000) and there may be periods of up to five years when no flow is released. 'It appears that closure of the mouth is likely to be a reasonably regular occurrence in the future' (Close 1990, p. 71). The water released from Lake Alexandrina is always turbid, as wind turbulence within the shallow lake resuspends sediment; large numbers of cyanobacteria are also common in these waters. These releases occur as flows of brown turbid water through the estuary and to the sea, and appear to remain in the nearshore zone.
A current review of the operation of the 'Cap' (on diversions in the Murray-Darling Basin) suggests that the quantity of fresh water allowed to pass the barrages is critical to the health of the estuarine and Coorong lagoon environment. Whittington et al. (2000) stated that 'salinity is the over-riding determinant of the distribution of plant and animal communities in the Coorong' (and Murray Mouth). During prolonged times of no flow (e.g. the El Nino phase of the early 1980s), salinity rose from seawater levels at the mouth (34 parts per thousand) to four times this value at the far end of the Coorong. Closure of the mouth (and therefore lack of tidal movement) simply exacerbates the problem of lack of fresh water flow, through evaporation effects. Although the Murray Mouth and the Coorong lagoon are Ramsar sites, and since 12 December 2000 have been reserves within the South Australian system, the most significant player in their management may prove to be the Murray-Darling Basin Commission.
The Great Barrier Reef Marine Park Authority and the Murray-Darling Basin Commission are often cited as leading examples of integrated natural resource management programs. Through the published results of monitoring and investigation within these programs it has been possible to indicate some of the impacts of catchment management on coastal areas. For most Australian coastal catchments this is not the case. As the 1996 State of the Environment Report for Australia noted (SEAC 1996), there is a lack of systematic monitoring of our catchment waters. The work on the catchments draining to the Great Barrier Reef Lagoon has shown (Wasson 1997) that detailed work may be needed in order to justify the expensive changes of management needed to reduce pollution.