series of cataracts on the Mekong River, extreme southern Laos, on the Cambodian border. The falls are the principal impediment to navigation of the river and have impeded economic use of the Mekong by the peoples of the Cambodian plain to the south and those of Laos to the north; a narrow-gauge railway was once built for transport around the falls. The double series of cataracts is caused by a resistant bed of basalt over which the river tumbles 45 feet (14 m) to a pool 269 feet (82 m) above sea level. The strata causing the falls are also responsible for several islands, the largest of which, Không, has a small port based on the portaging of goods around the falls. The Khone has the greatest volume of the world's waterfalls, its 2,500,000 gallons (9,500,000 litres) per second being nearly double that of Niagara Falls.