The Whangaehu River catchment covers an area of 1992 square kilometres. The mainstem of the Whangaehu River is 202 kilometres with a total stream length in the catchment of 3326 kilometres. The Whangehu catchment has two main tributaries; the Mangawhero (126 kilometres) and the Makotuku (36 kilometres) Rivers. The headwaters of the Wangaehu River are the crater lake of Mount Ruapehu on the Central Plateau. The river enters the Tasman Sea 8 kilometres southeast of Wanganui.