The Peel-Harvey Catchment encompasses an area of approximately 1,147,374 ha around the Serpentine, Harvey and Murray River systems (the latter includes the Hotham and Williams Rivers) that drain into the Peel Inlet and Harvey Estuary south of Mandurah. The catchment extends up to 170 kilometres inland and is 100 km north to south in the area closest to the coast. (Wells, 2005)
(Adapted from data provided by the Department of Agriculture)
The Peel Inlet and Harvey Estuary is a Ramsar listed wetland of international significance into which the majority of the catchment drains. The Peel Inlet has a surface area of 75 km2 and a volume of approximately 6l million cubic metres (61 000 megalitres) while the Harvey Estuary has a surface area of 61 km2 and a volume of approximately 56 million cubic metres (56 000 megalitres). The average depth of the combined system is 90 centimetres. Two openings connect the inlet to the ocean. The natural connection is located at the north of the Peel Inlet at Mandurah and is approximately 4.5 kilometres long. The Dawesville cut connects the estuary to the ocean at the junction of the Peel Inlet and Harvey Estuary. The 2 kilometre long man made channel was opened in 1994 in response to deteriorating water quality in the system.
The catchment is made up of two Physiographic regions. The coastal plain area around the inlet is generally low, lying, flat land and extends 20 to 30 kilometres inland. Much of this area is, or was prone to inundation, and wetlands such as those in the southern Serpentine Catchment, were prevalent prior to agricultural development. The majority of this area is less than 20 metres above sea level and slopes are generally extremely gentle. The Darling Plateau lies immediately to the east of the coastal plain. The Darling Scarp marks the edge of the plateau where elevation increases dramatically, in some cases by 400 metres in the space of 10 kilometres. This area on the western portion of the plateau is characterised by steep slopes and deeply incised valleys. In areas further east on the plateau, valleys tend to be broader with flatter floors. Elevation continues to increase although more gradually on the scarp. (Bussemaker, 2004)