The O'Shannassy River rises within the Yarra Ranges National Park on the slopes of the Yarra Ranges, within the Great Dividing Range, below Mount Observation, approximately 80 kilometres (50 mi) east of Melbourne. The river flows generally south by southwest, joined by two minor tributaries, before being impounded to create the O'Shannassy Reservoir, part of Melbourne's water supply system. Below the reservoir the natural flow of the river is diverted via the O’Shannassy Aqueduct and then reaches its confluence with the Yarra River west of the locality of McMahons Creek.[4][5] The river descends approximately 406 metres (1,332 ft) over its 15-kilometre (9.3 mi) course, before being diverted via the aqueduct.[3]
The O’Shannassy River catchment is identified as an Essentially Natural Catchment by the Land Conservation Council.[6]
