Niagara Falls is three waterfalls that straddle the international border between the Canadian province of Ontario and the US state of New York. They form the southern end of the Niagara Gorge.

From largest to smallest, the three waterfalls are the Horseshoe Falls, the American Falls and the Bridal Veil Falls. The Horseshoe Falls lie on the border of the United States and Canada while the American Falls lie entirely on the United States' side, separated by Goat Island. The smaller Bridal Veil Falls are also on the United States' side, separated from the American Falls by Luna Island.

Located on the Niagara River, which drains Lake Erie into Lake Ontario, the combined falls form the highest flow rate of any waterfall in North America that has a vertical drop of more than 50 metres (160 ft). During peak daytime tourist hours, more than 168,000 m3 (six million cubic feet) of water goes over the crest of the falls every minute. Horseshoe Falls is the most powerful waterfall in North America, as measured by flow rate.

The falls are 27 kilometres (17 mi) north-northwest of Buffalo, New York, and 121 kilometres (75 mi) south-southeast of Toronto, 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 (the last ice age), and water from the newly formed Great Lakes carved a path through the Niagara Escarpment en route to the Atlantic Ocean.

Niagara Falls is famed both for its beauty and as a valuable source of hydroelectric power. Balancing recreational, commercial, and industrial uses has been a challenge for the stewards of the falls since the 19th century.

The Horseshoe Falls drop about 57 metres (187 ft), while the height of the American Falls varies between 21 and 30 metres (69 and 98 ft) because of the presence of giant boulders at its base. The larger Horseshoe Falls are about 790 metres (2,590 ft) wide, while the American Falls are 320 metres (1,050 ft) wide. The distance between the American extremity of the Niagara Falls and the Canadian extremity is 3,409 feet (1,039 m).

The peak flow over Horseshoe Falls was recorded at 6,400 cubic metres (230,000 cu ft) per second. The average annual flow rate is 2,400 cubic metres (85,000 cu ft) per second. Since the flow is a direct function of the Lake Erie water elevation, it typically peaks in late spring or early summer. During the summer months, at least 2,800 cubic metres (99,000 cu ft) per second of water traverses the falls, some 90% of which goes over the Horseshoe Falls, while the balance is diverted to hydroelectric facilities. This is accomplished by employing a weir – the International Control Dam – with movable gates upstream from the Horseshoe Falls. The falls' flow is further halved at night, and, during the low tourist season in the winter, remains a minimum of 1,400 cubic metres (49,000 cu ft) per second. Water diversion is regulated by the 1950 Niagara Treaty and is administered by the International Niagara Board of Control (IJC).

The verdant green colour of the water flowing over the Niagara Falls is a byproduct of the estimated 60 tonnes/minute of dissolved salts and "rock flour" (very finely ground rock) generated by the erosive force of the Niagara River itself.

Peak visitor traffic occurs in the summertime, when Niagara Falls are both a daytime and evening attraction. From the Canadian side, floodlights illuminate both sides of the falls for several hours after dark (until midnight). The number of visitors in 2007 was expected to total 20 million, and by 2009 the annual rate was expected to top 28 million tourists.

The oldest and best known tourist attraction at Niagara Falls is the Maid of the Mist boat cruise, named for an ancient Ongiara Indian mythical character, which has carried passengers into the rapids immediately below the falls since 1846. Cruise boats operate from boat docks on both sides of the falls, with the Maid of the Mist operating from the American side and Hornblower Cruises from the Canadian side.

From the U.S. side, the American Falls can be viewed from walkways along Prospect Point Park, which also features the Prospect Point Observation Tower and a boat dock for the Maid of the Mist. Goat Island offers more views of the falls and is accessible by foot and automobile traffic by bridge above the American Falls. From Goat Island, the Cave of the Winds is accessible by elevator and leads hikers to a point beneath Bridal Veil Falls. Also on Goat Island are the Three Sisters Islands, the Power Portal where a huge statue of Nikola Tesla (the inventor whose patents for the AC induction motor and other devices for AC power transmission helped make the harnessing of the falls possible) can be seen, and a walking path that enables views of the rapids, the Niagara River, the gorge, and all of the falls. Most of these attractions lie within the Niagara Falls State Park.

The Niagara Scenic Trolley offers guided trips along the American Falls and around Goat Island. Panoramic and aerial views of the falls can also be viewed by helicopter. The Niagara Gorge Discovery Center showcases the natural and local history of Niagara Falls and the Niagara Gorge. A casino and luxury hotel was opened in Niagara Falls, New York, by the Seneca Indian tribe. The Seneca Niagara Casino & Hotel occupies the former Niagara Falls Convention Center. The new hotel is the first addition to the city's skyline since completion of the United Office Building in the 1920s.

On the Canadian side, Queen Victoria Park features manicured gardens, platforms offering views of both the American and Horseshoe Falls, and underground walkways leading into observation rooms that yield the illusion of being within the falling waters. The observation deck of the nearby Skylon Tower offers the highest view of the falls, and in the opposite direction gives views as far as Toronto. Along with the Minolta Tower (formerly the Seagrams Tower and the Konica Minolta Tower, and since 2010 called the Tower Hotel), it is one of two towers in Canada with a view of the falls.

Along the Niagara River, the Niagara River Recreational Trail runs 35 miles (56 km) from Fort Erie to Fort George, and includes many historical sites from the War of 1812.

The Whirlpool Aero Car, built in 1916 from a design by Spanish engineer Leonardo Torres y Quevedo, is a cable car that takes passengers over the Niagara Whirlpool on the Canadian side. The Journey Behind the Falls consists of an observation platform and series of tunnels near the bottom of the Horseshoe Falls on the Canadian side.

There are two casinos on the Canadian side of Niagara Falls, the Niagara Fallsview Casino Resort and Casino Niagara.

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