MARGAREE RIVER, Nova Scotia The Margaree is Cape Breton's River. The southwest branch begins at the largest lake on Cape Breton Island, Lake Ainslie. The northeast arm rises in the wilds of the Cape Breton Highlands. The two branches meet, and the main stem river proceeds through deep salmon pools and across floodplains, en route to the Gulf of St. Lawrence.The Margaree Valley has among the richest and oldest cultural heritage in Canada; Acadian and Scot. The Margaree River shows itself off best in autumn, when it is framed by the vibrant reds and oranges of the hardwood forests.