The Avon basin is the largest in South West WA, covering about 120,000 km2. It is climatically and geologically diverse with lower gradients and a more arid climate in the east (upper catchment) and higher gradients and a more temperate climate in the west (lower catchment). Consequently, rivers to the east have a slower and more periodic flow regime, and rivers to the west are faster-flowing and typically more incised. Salt lakes are common throughout the eastern catchments.
Average annual rainfall in the catchment is higher near the coast and in Perth is about 730 mm, rising to 1180 mm at Bickley on the Darling Scarp. Further inland the climate is warmer and drier. Merredin, in the centre of the catchment (260 km east of Perth), lies in the paleo drainage catchment of the Yilgarn River and receives an annual average rainfall of just 325 mm.
With much of the Avon basin being in the WA wheatbelt, around 64,000 km2 of land has been cleared (51% of total catchment) and is predominantly used for broadacre cropping. Nutrient enrichment from fertiliser run-off and secondary salinisation due to the widespread, historical land clearing are the primary pressures on the river and its tributaries. The Dale River catchment is a notable exception due to the influence of relatively fresh groundwater discharges. A river training scheme, conducted between 1956-72 to address flooding, also had the unintended consequence of mobilising sediments throughout the Avon River and the resultant sedimentation has reduced the capacity of important refuge pools which, along with increased salinity and eutrophication, threaten the ecological health of the river.