The river flows in a generally northerly direction from Tullnawood lake, past Darkley, passing within a few miles of Keady town, through Tassagh and on to Armagh. The Callan River, flowing by the City of Armagh, and the Tall River, running by Richhill, join together, and the united steam enters the Blackwater one mile below Charlemont
The area upstream of Armagh has many former linen mills that were built to use the waterpower of the river. There is also a viaduct at Tassagh that once carried the railway line from Armagh to Keady.
- Niall Caille Mac Aedo (791-846), High King of Ireland, who had defeated the Danes on the River Foyle in 833 and at Lough Swilly in 843, drowned in the River Callan, near Armagh.