Rivers

The Avon River is typical of rivers in the West Australian Wheatbelt but differs significantly from other river systems in that the gradient of the river channel increases downstream, the channel width and floodplain decrease downstream and the annual rainfall gradient increases away form the source. It is a river system that drains an ancient plateau landscape through the uplifted Darling Scarp to the Swan Coastal Plan where it is renamed the Swan River. The Avon/Swan River has a total catchment area of 125,000 km² extending from Dalwallinu in the north, Southern Cross in the north-east and Lake King in the south east down to the mouth at Fremantle. The Avon River Basin is Australian Water Resource Council (AWRC) basin number 615.

The Avon River Basin east of the 'Meckering Line' (the eastern axis of topographic uplift to form the Darling Scarp) is an ancient and very flat landform (the Yilgarn Plateau). It is thought that drainage from the plateau was originally to the Southern Ocean but that uplift of the South Coast landform (the 'Ravensthorpe Ramp') reversed flow to the north. It is also thought that the Avon may have discharged to the Hotham River at one stage but was 'captured' by the Swan River that eroded through the Darling Scarp near Toodyay.

The major tributaries to the Avon River are the Lockhart and Yilgarn Rivers. Both are characterised by chains of lakes and intermittent streamflow. The confluence of these tributaries is at the 'Caroline Gap' south of Kellerberrin from where the Salt River flows south-west to the Yenyenning Lakes and into the Avon River. The Yilgarn River, which originates north-east of Southern Cross from Lake Seabrook and Lake Deborah, drains an area of 55 900 km². The Lockhart Catchment is an area of 32 400 km² drained by three major rivers:

• The Lockhart River originating from Lake Magenta.
• The Camm River draining from Lake King through Hyden to Kondinin where it meets the Lockhart River.
• The Pingrup River which arises near Lake Cairlocup and flows north to Lake Grace and the Lockhart River.

Stream flow measured in the Yilgarn and Lockhart Rivers since 1976 shows there to be annual flow through these systems although flow volume is highly variable. Annual flow in the Lockhart River was in excess of 10 GL in seven out of the 25 years of record, while in 10 years it was less than 1 GL.

The main channel of the Avon River originates at Yealering Lakes (wetlands of national significance) upstream of the confluence with the Salt River at Yenyenning Lakes and drains a catchment area of 3 200 km². Downstream from the Yenyenning Lakes, it flows past Beverley and York to the confluence with the Mortlock River at Northam. The catchment area of the Avon River at Northam is 8 100 km², while the Mortlock River adds a further 16 800 km². The Yenyenning Lakes are a series of lakes less than one kilometre in diameter but with a combined length of 15 km. The bed of the lakes system has very low gradient (1:20 000). Construction of a permanent causeway with culverts at the Avon River confluence (Qualandary Crossing) provides a controlled outlet so that water levels can be maintained for recreation use and water birds, and release of hyper-saline water from the lakes can be controlled.

The Brockman River, Ellen Brook, Helena River and Wooroloo Brook are also tributaries to the Avon/Swan river system. Management of natural resources within these two catchment areas is coordinated through the Swan Catchment Council.

Wetlands

The aquatic ecosystems of the region comprise of fresh and saline wetlands and salt lakes. Most freshwater wetlands are located in the 'Avon Arc' Zone with several significant fresh wetlands in the Wheatbelt and Rangelands areas. A number of the region's fresh and saline wetlands are of national or sub-regional significance with a large number of other wetlands identified as important. There are extensive 'chains' of salt lakes in the Avon River Basin, with the Lake Grace System being of national significance but their hydrological and ecological processes are not well understood.