When there is less rainfall than normal, the water level of Australian rivers falls and the rivers flow more slowly  Because algae whose habitat is river water grow best in slow-moving water, the amount of algae per unit of water generally increases when there has been little rain
By contrast, however, following a period of extreme drought, algae levels are low even in very slow-moving river water
Except during periods of extreme drought, algae levels tend to be higher in rivers in which the flow has been controlled by damming than in rivers that flow freely.