Formed between 10,000 and 12,000 years ago when the retreat of melting glaciers allowed water trapped in Lake Erie to flow northward to Lake Ontario, Niagara Falls is between 161 and 167 feet high (49-51 m). Rapids precede the falls for seven miles (11 km). Goat Island divides the Niagara River into the Horseshoe Falls on the Canadian side and American Falls on the U.S. side. In 1969 the river was temporarily diverted to permit studies of methods of slowing the erosion that had been eating away at the falls over the centuries.