The Shannon River catchment is 3368 km2 and incorporates the southern Darling Plateau and parts of the Ravensthorpe Ramp and Scott Coastal Plain.
Three main rivers, each less than 50 km in length, are present: the Gardner (discharging directly to the ocean), the Shannon (discharging to Broke Inlet) and the Deep (discharging to Walpole-Nornalup Inlet). This region has a highest rainfall in south-west Western Australia, in excess of 1150 mm/yr in the south-western corner and along coastal margins, but decreasing to 700 mm in the northern section (mean annual rainfall 1975–2003).
Only small areas of the Shannon River catchment are cleared for agriculture, with the majority of the catchment being covered in dense remnant vegetation. A large percentage of the Broke Inlet is protected by conservation estates (the remainder being managed resources and some horticulture), while most of the inland waters are fresh.