Its headstreams flow from the slopes of Broad Cairn and Cairn Bannoch and include the small corrie lochan of Loch Esk. In its upper reaches it is joined at Braedownie by the White Water which flows through Glen Doll. The river continues its south east course through Glen Clova and is joined near the mouth of the glen by the Prosen Water.
Flowing from the Braes of Angus into the wide valley of Strathmore, the South Esk meanders eastwards past Brechin before entering Montrose Basin, through which it flows outwards to the North Sea by a narrow channel between Montrose and Ferryden.
During the summer the South Esk is one of the best sea trout rivers in Scotland. With the right weather conditions salmon runs are good in the spring and excellent in the autumn. The river is some forty nine miles (79km) long with superb fishing from Glen Clova down to Montrose Basin where it then enters the sea.