The River Boyne, at in length, is Meath's dominant geographic feature and is synonymous with the county, having defined its history and culture over millennia. The two most prominent tributaries of the Boyne are the Leinster Blackwater, which has its source in Cavan and flows south for before joining the Boyne at Navan, and the Enfield Blackwater, which has its source in Kildare and flows north for before joining the Boyne at Donore. In the east of the county, both the River Nanny and the Delvin River flow to the Irish sea, with the latter demarcating the border with County Dublin.