Slow and Sluggish: Because their marine openings are so constricted, the Lakes have an extremely low rate of water circulation. It can take up to 2 years for water to be flushed into the sea, and in some inner bays and coves, far from the marine interface, the flushing rate slows to an astonishing 40 years! Although the year-round population of the Bras d’Or region is relatively small—less than 15,000 people – the Lake’s slow flushing rate means that small amounts of pollution can add up to big water quality problems.

Bras d’Or Basics
• The highly irregular shoreline of the Bras d’Or Lakes is 1,234 kilometres long. About 13% is rock and about 27% lies along the Lakes’ narrow channels.

• The “main lake” of the Bras d’Or, situated in the south-central part of Cape Breton, is connected to the Lakes’ channels by the small Barra Strait.

• The Great Bras d’Or Channel to the northeast (only 320 metres wide in its narrowest stretch) is the Lakes’ main source of seawater inflow and lakewater outflow. There is a net inflow of seawater in the subsurface layers and a net outflow of fresher water on the surface layer.

• The salinity of Bras d’Or surface waters varies from 29 parts per thousand at the entrance to Great Bras d’Or to 20 -21 parts per thousand at the east end of East Bay. Sheltered bays connected to rivers that drain into the western part of the Lakes have even lower salinity levels.

• There is no natural connection between the Lakes and the Atlantic Ocean to the southwest. A man-made connection has existed since 1869, when the St. Peter’s Canal was completed through an isthmus at St. Peter’s Bay.

• Salinity levels in the Lakes depend on rain run-off, depth, stream drainage and proximity to the northern sea connections.

• The Lakes are underlain by sedimentary rock, composed mainly of shale, sandstone, gypsum and salt. Deeper areas are floored by mud, with more exposed, shallow areas covered in gravelly glacial till.

• Tides in the Lakes range less than half a metre; tidal currents in the entrance to Great Bras d’Or average 4 -5 knots. Non-tidal currents are weak.

• Much of the Lakes are covered by ice in winter.