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Shannon River at a glance Shannon River view
Location:   19 kilometres west of Walpole
Catchment area:   1000 square kilometres
Average annual catchment rainfall:   1350 millimetres
River length:   65 kilometres
Annual discharge volume into inlet:   157M cubic metres
Extent of tidal influence:   Three kilometers
River health:   good - no salinity problems

The Shannon River drains forest in the 1000-1400 millimetres rainfall zone, to the west of the Deep River. After flowing through steep valleys in its upper reaches, the river crosses a wide swampy coastal plain before flowing into Broke Inlet. The Broke Inlet is a large estuary of 48 square kilometres which is closed from the sea by a sand-bar for most of the year. However, the bar breaches in most years, after which stays open for up to six months. The river water is both fresh and low in nutrients and sediments. The entire catchment and Broke Inlet is contained within the Shannon and D'Entrecasteaux National Parks.

Temperature and Salinity graph The wetland system is important to the survival of migratory birds, supports large numbers of waterbirds and is an excellent example of a south coast wetland. These wetlands are under threat from rising water tables, salinity and other issues as a result of land management practices in the upper catchment.

Sandbanks bar the mouth of the river, where it enters Broke Inlet. The river channels are deep, from three metres on straight areas up to ten metres where it is scoured on a bend. The inlet channel is up to six metres deep, with steep slopes on either side.

In summer, when the river flow is slight and the bar is open, high salinity water flows from the lagoon into the Shannon River under the river water, causing strong stratification. This corresponds strongly with the temperature profile of the water at the same date.

Some oxygen depletion (3.7 mg/litre) has been recorded in the deeper waters of the river.

In 2005 a project (Our Living Rivers) was set up to collect and analyse data, enabling the Department of Water to monitor the health of the river over time and help determine what systems need better protection. In spring 2006 and 2007 samples were taken from the river system to assess the quality of the water and habitat as well as the presence of fish and macroinvertebrates. Five sites were sampled in the Shannon River catchment.

Further Information:

Estuaries and Coastal Lagoons of South Western Australia. Broke Inlet and other estuaries of the Shire of Manjimup. Environmental Protection Authority; Perth, Western Australia. Estuarine Studies Series Number 6: September 1989

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