The river is one of two discharging in Kilkeel Harbour, the other being the Little Kilkeel River or Aughrim River.
The Kilkeel River rises on the slopes of Down's second-largest mountain, Slieve Commedagh in the northeast of the Mourne Mountains. From here it flows southwest, and is dammed to form the Ben Crom Reservoir, a mile and a half long. Below the dam the waters run for just over half a mile, by the name of the "Mill River" to enter the yet greater Silent Valley Reservoir, a broad and capacious water two miles long, north to south.
The Silent Valley and its forest park are a popular visitor attraction, and the Silent Valley and Ben Crom reservoirs provide the drinking water for Belfast and much of County Down.
Below the Silent Valley, liberated from the needs of the water company, the river leaves the mountains to tumble down due south through the farmland towards the coast, passing no villages on its way until Kilkeel and the sea.