Background
Between May and October of each year, about 1000 mm of rain fall into the catchment areas of the Murray, Serpentine and Harvey Rivers and their tributaries. This represents almost all of the annual rainfall, and so in the drier summer months the rainfall is all but non-existent. This is due to the region's Mediterranean climate.
The first European settlers had found the area almost impassable for many months of the year as the flatlands above the estuary become vast floodplains. During the late 1800s, landowners and governments started constructing major drainage systems and culverts to remove the excess water to free up grazing land for livestock and pastures. Forested areas were also cleared. Since then, over 100 years of development of the low-lying sandplain depleted the moisture holding capacity of the soil and any rainfall that fell in the region quickly dissipated into the drains. Soil quality became increasingly degraded and farmers reverted to excessive superphosphate use to compensate for the loss of nutrients.
In the early 1970s, several industries, including a large piggery and sheep holding paddocks within the lower catchment poured quantities of nutrients into the river system.
A tipping point occurred during the 1970s and 1980s when the discharge of nutrients into the estuary resulted in it becoming eutrophic. Growths of macroalgae in the form of toxic cyanobacteria nodularia spumigena started occurring on a regular seasonal basis. The combination of sunlight and stagnant, nutrient rich, heated water caused massive blooms of blue-green algae.
In the meantime, the region surrounding the estuaries, particularly the coastal strip south of Mandurah was undergoing a major population growth.
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