The River Dhoo is a river on the Isle of Man. The river begins in Marown and flows east towards Douglas through the central valley of the island. The river passes Crosby and Union Mills before meeting with the River Glass on the outskirts of Douglas where it flows out to sea through Douglas Harbour. The Dhoo (meaning black or dark in Manx) and the Glass (meaning clear or green) converge to form the River Douglas which some say gives Douglas its name. It has a length of approx.