The Lachine Rapids are a series of rapids on the Saint Lawrence River, between the Island of Montreal and the south shore. They are located near the former city of Lachine. The Lachine Rapids contain large standing waves because the water volume and current do not change with respect to the permanent features in the riverbed, namely its shelf-like drops. Seasonal variation in the water flow does not change the position of the waves, although it does change their size and shape. The rapids are about 3 miles (4.8 km) in length. In the past these represented a considerable barrier to maritime traffic. Until the construction of the Lachine Canal through Montreal, the rapids had to be portaged. Even with the canal, the difficulty was such that it was usually more convenient to ship goods by rail to Montreal, where they could be loaded at the city's port. Montreal remains a major rail hub and one of Canada's largest ports for that reason.
The Lachine Rapids are now passed by the South Shore Canal (Saint-Lambert and Côte Sainte-Catherine locks) of the Saint Lawrence Seaway.