This is the northernmost district of North Rhine-Westphalia. It protrudes into Lower Saxon territory. The Weser River enters the district in the southeast and leaves to the north. In the south of the district the river runs through a narrow gorge, which is formed by two mountain chains, the Wiehen Hills in the west and the Wesergebirge in the east. This gorge is called Porta Westfalica, since it marks the ancient border of Westphalia.