A Unique Inland Sea
Freshwater lake or saltwater sea?

A single body of water, or a rambling series of interconnected bays and channels?

In many respects, Nova Scotia’s 260-square-kilometre Bras d’Or Lakes are a natural wonder that defies categorization. The Lakes’ waters, a mixture of Atlantic seawater, local freshwater run-off and a small amount of fresh groundwater, are brackish. Furthermore, the Bras d’Or’s long, narrow arms (St. Patrick’s Channel, St. Andrews Channel and Great Bras d’Or Channel), “main lake” and innumerable bays and indentations are so tenuously linked that they could be regarded as separate geographical elements.

While the use of the plural term “Lakes” conveniently conveys the association of the waters that sprawl across Cape Breton, it is not quite as accurate in its oceanographic description. With an average salt content of 22 parts per thousand, compared to a salt content of 35 parts per thousand in the ocean, the waters of the Bras d’Or Lakes are more typical of mixed-water estuaries that link freshwater rivers with the saltwater sea.

Rising and Falling: The estuarine nature of the Bras d’Or Lakes is derived from their openings to the ocean – through the narrow Great Bras d’Or Channel and the very narrow Little Bras d’Or Channel to the northeast (on the Gulf of St. Lawrence), and through the man-made St. Peter’s Canal to the southwest. During the past 15,000 years, marked by dramatic glacial activity and geological upheaval, the salinity of the Lakes has varied, as sea levels rose and fell. About 9,000 years ago, the sea level dropped so much that the Lakes were completely cut off from the ocean. When sea levels rose again about 4,500 years ago, a limited marine exchange resumed in the northern channels, once again introducing saltwater to the inland freshwaters of the Bras d’Or.

Today, the far-flung waters of the Bras d’Or Lakes are characterized by contrast – in their salinity, currents, temperature, topography, depth, flushing rate and tidal reach. More like a watery scrawl, etched unevenly across the Cape Breton landscape, than a tidy, bowl-shaped water body, the Lakes range from a depth of only 16 metres in the Great Bras d’Or Channel to 280 metres in the St. Andrews Channel. The western part of the Lakes is generally shallow, with the “main lake” reaching a maximum depth of 157 metres. Some geologists have suggested that the irregular topography is partly due to the collapse of gypsum that underlies some parts of the Lakes.