The Bras d'Or Lake comprises an irregular brackish body of water covering 260 km2. The western part of the lake is generally shallow, with the sheltered bays of West Bay, Denys Basin, and Whycocomagh Bay. Three long narrow arms extend to the east: East Bay, St. Andrews Channel, and Great Bras d'Or Channel. Great Bras d'Or Channel connects to the open sea in the Sydney Bight across a depth of at least 8 m. Little Bras d'Or Channel is a 6-m deep, sinuous estuary that connects St. Andrews Channel with the sea. A narrow isthmus at St. Peters separates the southern part of Bras d'Or Lake from St. Peters Bay. There is no direct information on the bedrock geology beneath the Bras d'Or Lake. Extrapolation of observations along the shoreline and in the adjacent lowlands suggests that the lakes are largely underlain by Carboniferous Windsor Group sedimentary rocks, principally shale, sandstone, gypsum, and salt. A large negative gravity anomaly beneath West Bay suggests the presence of salt. The Bras d'Or Lake occupies a regional lowland that developed in soft Windsor Group rocks before the Quaternary glacial period. Some deepening of the floor of the lake might have resulted from solution collapse of gypsum, but the main excavation of the very deep channels (280 m in St. Andrews Channel, 81 m in East Bay) appears to be a consequence of glacial erosion, probably over hundreds of thousands of years through the Quaternary. The cliffs bordering the lake are unusual because they preserve organic sediments predating the last glaciations that provide a window on earlier environmental conditions. The morphology of the lake floor is influenced by the deposition of glacial till and pre-glacial silty muds that occurred during the last retreat of ice. The extensive drumlin field of southern Cape Breton Island extends across much of the central and western parts of the lake. Remaining ice appears to have been centered on the western part of Bras d'Or Lake, and a series of recessional moraines are visible on the floors of East Bay, St. Andrews Channel, and Great Bras d'Or, with pre-glacial silty muds thickening eastward. The shallowness of the links between the Bras d'Or Lake and the Atlantic Ocean have resulted in a complex post-glacial history. Final melting of glacial ice probably occurred about 10,000 years after the Younger Dryas climatic oscillation. The first sediments deposited above glacial till in the central part of the lake, probably 10,000 to 9,000 years ago, contain dinocysts that indicate some penetration of marine water into the lake. The relatively high sea level inferred at this time reflects the continuing depression of the land from loading by glacial ice. Rebound from this depression cut off marine-water influx from about 9,000 to 4,500 years ago and the Bras d'Or Lake was fresh. Late Holocene subsidence resulted in a renewed influx of marine water in the last 4,500 years. The effects of this subsidence are seen in the transgressive character of many of the shoreline features and the extensive shoals of upper Whycocomagh Bay, Nyanza Bay, and Denys Basin, which lay at the mid-Holocene lake shoreline. Sediment distribution in the Bras d'Or Lake is similar to that found in many of the larger coastal inlets on the southern shore of Nova Scotia. Deeper areas of the lake are floored by mud, except for the sands found in some areas flushed by tidal currents. More exposed shallow areas of the western part of the lake are commonly floored by gravelly, sandy mud that resulted from the erosion of glacial till. Coastal erosion of glacial sediments has led to the formation of many sandy and gravelly barrier beaches and spits. Bras D'Or Lake is a fiordal system connected to the sea via two restricted channels. This restricted access causes the tidal amplitude to be reduced and, in combination with high freshwater runoff, results in relatively low salinity. The salinity of surface waters vary from about 29 p.p.t. at the entrance to Great Bras d'Or, to 25-26 in the deep water basins, to 20-21 in surface waters at the east end of East Bay. Lower salinities are found in sheltered bays off the larger rivers that drain into the western part of the lake. A thermocline and halocline develop at 10-20 m during the summer and probably deepen in the winter. Measurements of oxygen and salinity indicate that lake water is a mix of Atlantic water and local runoff, with an insignificant contribution from groundwater. Most of the lake is covered by ice in winter, with temperatures warming by more than 10°C from May to July. The Bras d'Or Lake shows a typical estuarine circulation, with brackish near-surface waters tending to flow seawards, and deep saline water tending to flow into the lake. Tidal currents in the entrance to Great Bras d'Or are normally 4-5 knots but reach 6 knots or more when the lake level is elevated by up to 30 cm during spring runoff or after northeast gales. Non-tidal flows in the lake proper tend to be very weak but in narrow passages between basins may reach about one knot. The long fetches in the eastern arms of the Bras d'Or Lake allow sizeable waves and swells to develop during northeast gales Seaweed species are similar to those of the Gulf of St. Lawrence. In both areas, seaweeds usually found in intertidal zones occur only in deeper water as the result of winter ice activity, and the rockweed Ascophyllum nodosum is found subtidally. Sheltered bays have marginal salt to freshwater marsh vegetation. The Bras d'Or Lake is one of the areas where the American Oyster is found, owing to warmer water temperatures suitable for growth and reproduction. A significant population of sand shrimp, a southern species, exists here. The polychaete fauna is Virginian in character but also includes some arctic-boreal species. A varied fish fauna includes Blueback Herring, Black-spotted Stickleback, and a southern population of Greenland Cod. A feral population of Rainbow Trout is present in the lake as well. These support strong populations of Great Blue Heron, Double-crested Cormorant, and Bald Eagle. Most cultural use of the Bras d'Or Lake is related to shore-based activities. The whole area is of high cultural significance to the Mi'kmaq people and is a center of Scottish heritage in Nova Scotia. The marine area has some natural fisheries, but aquaculture for oysters and salmonids is most important. The area is important for recreational boating.