The Bras d'Or Lake is an estuarine system located in the middle of Cape Breton Island. Cape Breton is a rugged and irregularly shaped island covering about . It lies northeast of mainland Nova Scotia, and is joined to the mainland by a causeway constructed in 1955. Differential erosion has resulted in steep hills around the lake and peninsulas within it, which divide the estuarine ecosystem into five long, deep channels in the north half of the lake and a number of bays in the south. Along the west side of the watershed, steep hills rise abruptly to highland plateaus (the Cape Breton Highlands) at elevations of .