The Great Ouse is now contained by flood banks and from these banks there are views of the surrounding countryside. Denver Sluice, being at the confluence of five watercourses, was first built across the river in 1651 as a focus of the flood defence system that protects the low lying Fens although it had to be rebuilt after bursting in 1713. Nearby Denver Windmill is a fully restored 19th century windmill, and lies on the path of the Roman Fen Causeway. The Ouse Washes are an internationally significant environment. Flooded in winter, they attract thousands of migrating wildfowl. From Denver the river is tidal, bringing with it subtle changes in scenery and habitat. Close by is the Relief Channel, the final link in the drainage system, completed in 1964. The two waterways meet at King's Lynn, the historic port on the edge of the Wash where the footpath ends.