The Blackwater or Munster Blackwater (, The Big River) is a river which flows through counties Kerry, Cork, and Waterford in Ireland. It rises in the Mullaghareirk Mountains in County Kerry and then flows in an easterly direction through County Cork, through Mallow and Fermoy. It then enters County Waterford where it flows through Lismore, before abruptly turning south at Cappoquin, and finally draining into the Celtic Sea at Youghal Harbour. In total, the Blackwater is 169 km (105 mi) long. The total catchment area of the River Blackwater is 3,324 km². The long term average flow rate of the River Blackwater is 89.1 Cubic Metres per second (m³/s)