The Glenelg River rises at an elevation of
above sea level below The Chimney Pots within the Grampians National Park, on the eastern slopes of the Victoria Range, and west of the Serra Range, within the Grampian Range. The river flows north through swampland before heading west, transversed by the
Henty Highway, and then south where the river is impounded by the
Rocklands Reservoir, formed by a concrete-walled
gravity dam with embankment sections constructed in 1953. The
reservoir has a maximum capacity of . Leaving the dam wall, the river flows west through
state forestry areas towards , where the river is joined by the Salt, Mather, Yarramyljup, and Schofield Creeks, flowing to the east, north, then west of Balmoral, through the settlement of , where the river is met by another creek, also called Salt Creek. The
Chetwynd River, draining the region north of and , joins the Glenelg east of Burke Bridge. The Glenelg flows generally southwest, to the west of Dergholm State Park towards Dergholm, joined by a number of minor tributaries. From here the river flows south by east, through the town of Casterton, and south of which the
Wannon River forms its
confluence with the Glenelg. Flowing south by west, the Glenelg flows through the Wilkin Flora and Fauna Reserve before heading due south, met by the
Stokes River prior to flowing through the town of where the
Crawford River forms its confluence with the Glenelg. Flowing south, then sharply west and crossing the state border into South Australia, the Glenelg flows through before heading east back into Victoria. The river reaches its
mouth at Nelson and much of the latter
course is through the Lower Glenelg National Park. From its highest point, the Glenelg River descends , joined by thirty-two named
tributaries over its course. == Flora and fauna ==