Although similar to Lake Erie in surface area and physical dimensions, Lake Ontario is a much deeper lake, averaging 283 feet. Comparing their volumes, Lake Ontario holds almost four times more water than Lake Erie. Ontario ranks fourth among the Great Lakes in maximum depth, but its average depth is second only to Lake Superior.
- Lake Ontario lies 325 ft (99 m) below Lake Erie, at the base of Niagara Falls.
- Niagara Falls were always an obstacle to navigation into the upper lakes until the Trent-Severn Waterway, along with the Welland and Erie canals were built to allow ships to pass around this bottleneck.
- The oldest lighthouse on the U.S. side of the Great Lakes was set up at Fort Niagara in 1818 to aid navigation.