The river has many tributaries at its upper end, the principal one being the River Bull; and its main channel begins at Hellingly. After crossing the Low Weald area of farmland, the Cuckmere cuts through the South Downs in its own valley. It reaches the English Channel at Cuckmere Haven, between Seaford and the Seven Sisters cliff face. The lower part of its course in the floodplain is marked by meandering. The Cuckmere Valley Nature Reserve is located in the lower estuary portion of the river. The valley is very important for nature conservation. It has been designated a Site of Special Scientific Interest and is part of the South Downs National Park.