The River Lee is a river in Ireland. It rises in the Shehy Mountains on the western border of County Cork and flows eastwards through Cork, where it splits in two for a short distance, creating an island on which Cork's city centre is built, then passes through Cork Harbour on the south coast, one of the largest natural harbours in the world, to empty into the Celtic Sea. The catchment area of the River Lee is 1,253 km2. The long-term average flow rate of the River Lee is 40.4 cubic metres per second (m3/s).