Beyond Lake Ontario, magnificent Niagara Falls blocked the way to the four remaining Great Lakes. In 1829, the first Welland Canal was opened, with a rise of 326 feet (99 meters) to bypass the Falls and reach Lake Erie. More than a century later, an improved Welland Canal made possible the longest artificial seaway in the world, the St. Lawrence–Great Lakes waterway.

Other concerns focus on the imbalance man-made waterways can cause in the natural creation. The Erie and Welland Canals opened the way for the sea lamprey, an eellike native of the North Atlantic, to invade the Great Lakes, where it decimated populations of commercially valuable fish.