Boulogne is at the mouth of the Liane river , which meanders through a valley between hills .
Boulogne - sur - Mer , Calais , Dunkirk and Dieppe , are Channel Ports on the French side at the narrowest part of the English Channel .
Boulogne - sur - Mer is at the mouth of the Liane river , which meanders through a valley between hills .
Boulogne - sur - Mer , Calais , Dunkirk and Dieppe , are Channel Ports on the French side at the narrowest part of the English Channel .