The lower reaches of the Swan and Canning rivers form an estuary created by geological conditions more than 10,000 years ago. The Swan River was mostly brackish before settlers removed the Fremantle bar in the late 1800s and dredged the large flood delta nearby. The Swan and Canning rivers are now a permanently open estuary that changes from fresh/brackish conditions in winter and spring, to salty conditions during summer and autumn.

The Swan Canning river system is typified by a large urban and agricultural catchment and relatively shallow and slow-moving river conditions. These factors as well as sandy soils and a climate of diminishing rainfall and long hot summers make the system vulnerable to a suite of environmental issues.

Swan and lower Canning rivers

A dominant feature of the estuarine hydrodynamics in the Swan and lower Canning rivers is a wedge of saline water that moves upstream from the ocean according to tidal and barometric forcing. Saline water is dense and tends to sit on the bottom of the river. When tides are strong and active the wedge travels upstream along the bottom of the river, while less dense fresh water flows downstream over the top.

The hydrodynamics change with season and rainfall. In early spring, when rainfall runoff and river flow decline, sea water assisted by tides moves progressively upstream to reach Middle Swan and Kent Street in the Canning River during late spring and early summer. The wedge often remains up to 55km upstream through summer and into late autumn. In very dry years substantial salty water can remain up river over the winter period.

In later autumn, as rains arrive and freshwater runoff increases, layering of freshwater over heavier, denser saltwater is common in the river. This salinity stratification restricts the mixing of the bottom waters with the surface waters, preventing oxygen replenishment from surface to bottom. Such conditions lead to a situation where bottom waters become hypoxic (low oxygen) and even anoxic (no oxygen) as the decomposition of organic matter at depth depletes oxygen and there is insufficient replenishment from the surface. Under anoxic conditions, the sediments may release nutrients which accumulate in the stagnant salty bottom water and contribute to algal blooms.

In late winter and early spring the system is generally at its most fresh and the downstream extent of the saline wedge will depend on the amount of rainfall received.

Climate change is decreasing rainfall and increasing tidal heights and storm surge in the river system. This is impacting on the hydrodynamics of the river system.

Upper Canning River

The upper Canning River is separated from tidal flow and marine water by the Kent Street Weir, creating a weir pool upstream. During winter, the weir boards are removed to allow water to flow downstream and reduce the potential for flooding upstream. In summer, the boards are replaced and the upper Canning River behaves like a freshwater pool. Stratification is often driven by temperature in this zone. Increasing tidal heights, storm surge and ageing weir infrastructure means that saline water is increasingly introduced into this zone, which has implications for habitat upstream of the weir.

Parks and Wildlife, in collaboration with the Department of Water, monitors the physico-chemical conditions in the Swan and Canning rivers. Weekly profiles of conditions in the Swan and Canning rivers are used as part of an estuarine water quality monitoring program.