Bras d'Or Lake Biosphere Reserve is a UNESCO Biosphere Reserve located at Bras d'Or Lake, Cape Breton Island, Nova Scotia, Canada. It consists of a salt-water estuary watershed inland sea with three passages to the Atlantic Ocean. The Holocene transgression flooded a complex river-lake system of diverse geology, creating the small, deep inland sea with 12 significant watersheds draining both highlands and lowlands.