Niagara Falls is one of the most viewable and well-visited sights in existence. They stretch across the Niagara River between the US and Canada, interrupted by Goat Island. Just over a kilometer wide, they carry 100,000 liters of water a second. They are over 164 feet / 50m high and are the most significant barrier on the short river, flowing from Lake Erie to Lake Ontario. Since the end of the last ice age, they have been there and have existed in their present form for about 5500 years since water scoured out an old riverbed and created the distinctive whirlpool seen today.
The falls have been eroding their way upstream since the end of the last ice age. In the last few hundred years, this rate of the recession has increased to around 3.28-4.92 feet / 1–1.5m a year, and, left to itself, the process would accelerate further in the future when the falls wear their way back to some softer rock upstream. However, engineers are working to slow this erosion and reduce the falls’ retreat to only about 0.98 feet / 30cm a year. Without this intervention, the falls would eventually work their way back to Lake Erie and cease to exist in about 50,000 years.