Saline Pools in Rivers
Saline pools in rivers are accumulations of saline ground water in the bottom of the deeper sections of rivers. Some of the worst examples occur in the Wimmera River, where, during summer, pools that look magnificent and have superb fish cover and riparian vegetation, are devoid of aquatic life.
The bottom metre or so of water can be saltier than seawater and reek of sulphur dioxide from decaying organic matter. Saline pools occur in areas where rising water tables are causing salinity problems and the first places to be affected are obviously the lowest points in the catchment i.e. the rivers and drainage lines.
The process of saline ground water intruding into the river is opposed by two factors. One is that the hydrostatic pressure of the water in the river tends to counteract the upward pressure of the ground water, and reduces, or prevents intrusion of saline water into the river. The other is that when flows are high, any saline water that intrudes is quickly moved downstream and diluted by the significantly greater volume of the fresh water. When flow decreases, both these factors are also reduced and eventually, the flow may be insufficient to move the heavier saline water out of the bottom of the deeper pools. Water in the bottom of the pool then becomes isolated from the surface water and very quickly becomes anaerobic.
The saline water prevents the bottom of the pool being used by fish or any other biota that need oxygen This condition persists until flows increase in the winter, and dilutes the saline water, or moves it downstream.
Saline pools occur naturally in some rivers. In others they are caused, or exacerbated, by unnaturally low summer flows due to water extraction, or to water tables rising due to land clearing and inappropriate land management. Programs to prevent land degradation are the first step in addressing this problem.