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Broke Inlet at a glance
Location:   19 kilometres west of Walpole
Catchment area:   928 square kilometres
Average annual rainfall over catchment:   1350 millimetres
Percentage catchment cleared:   4.5%
Inlet area:   4800 hectares
Inlet volume:   72M cubic metres
Annual flow into Broke Inlet:   187M cubic metres
Flow to estuary:   157M cubic metres
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       Broke Inlet view

Broke Inlet is a long, shallow lagoon, with a sea bar closed for half the year, and its major inflow from the Shannon River. The inlet water is brackish, seldom more than half sea water salinity. Water level can vary from two metres above mean sea level before the bar breaks to one metre below at the end of summer.

Surface salinity varies from around 5 ppt when the river is flowing in summer to 30 ppt or more at the end of summer, but both lower and higher measurements (up to sea water, at 35 ppt) have been observed. There is great variation from year to year depending on the volume of the river and the time and duration of the bar opening. Mixing is rapid in the open water of the lagoon, and the salinity is generally uniform throughout the lagoon and from top to bottom, except when the river is flowing strongly.

Observed water temperature in the inlet varies from 11 to 22°C. The clarity of the water varies greatly, depending on the degree of wind. The surface water is well oxygenated, and this is probably true for all the water in the lagoon.

Sampling has shown uniformly low levels of total phosphorus, most 0.01 mg/litre or less. Limited sampling has also shown low nitrogen levels (<0.5 mg/litre Total N and 0.02 mg/litre nitrate).

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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