The Irwin River is located in the Greenough Basin. It is 160km long and has a catchment of 6,072km2. The river system starts east of Mullewa. The middle and lower reaches of the river are perennial due to discharge from groundwater systems. The river enters the ocean at the Irwin River mouth, in Dongara. The mouth of the river is a coastal lagoon and wetland system, and only opens to the ocean during higher river flows in winter.