 The catchment includes part of the Swan Coastal Plain, the Darling Scarp and the Darling Plateau.The Collie and Preston Rivers are the main catchment rivers and enter the estuary from the southern end and with the catchment runoff discharging into the Indian ocean via the cut in the peninsula.Other rivers in the catchment area include the Brunswick River, Ferguson and Wellesley as well as numerous other creeks, streams and irrigation drains.