The Maruia River rises in the Spenser Mountains, travelling first to the southwest before turning north for the last 50 km of its length. In its upper reaches, the river's valley forms the western approach to the Lewis Pass, the northernmost of the three main mountain passes across the Southern Alps. Hot springs are to be found close to the river in its upper reaches, and the spa of Maruia Springs is located five kilometres to the west of the Lewis Pass, 50 km southeast of Reefton.