Niagara Falls is a breathtaking group of three waterfalls that span the border between the Canadian province of Ontario and the United States state of New York. The largest of the three is Horseshoe Falls, also known as the Canadian Falls, which straddles the international border of the two countries. The smaller American Falls and Bridal Veil Falls lie entirely within the United States. Niagara Falls is formed by the Niagara River, which drains Lake Erie into Lake Ontario, and it has the highest flow rate of any waterfall in North America that has a vertical drop of more than 50 m (160 ft). During peak daytime tourist hours, more than 168,000 m3 (5.9 million cu ft) of water goes over the crest of the falls every minute. The verdant green color of the water flowing over Niagara Falls is a byproduct of the estimated 60 tonnes/minute of dissolved salts and rock flour generated by the erosive force of the Niagara River. The falls are not only a beautiful tourist destination, but also a vital source of hydroelectric power. The challenge of balancing recreational, commercial, and industrial uses has been faced by the stewards of Niagara Falls since the 19th century. Niagara Falls is located 27 km (17 mi) northwest of Buffalo, New York, and 69 km (43 mi) southeast of Toronto, Ontario, between the twin cities of Niagara Falls, Ontario, and Niagara Falls, New York. Niagara Falls was formed when glaciers receded at the end of the Wisconsin glaciation, and water from the newly formed Great Lakes carved a path over and through the Niagara Escarpment en route to the Atlantic Ocean. With Horseshoe Falls being the most powerful waterfall in North America, Niagara Falls is a must-see destination for any traveler visiting Canada or the United States.