Most waterfalls occur in a places where the path of a river crosses a vertical drop in ground level. There is one waterfall, possibly the only one in the world, where the drop is parallel to the course of the river: Moconá Falls, on the Uruguay River that runs along the border of Argentina and Brazil. These falls run sideways, but it’s only a “waterfall” about half of the year! How does that happen?
An unusual feature of the Uruguay River is the presence of a submerged canyon or trench at the bottom of the river channel. The canyon, which is believed to have formed during the Ice Age, when the climate was drier and the river was narrower, is up to 100 meters deep and 15 - 30% of the width of the river. The canyon is only visible in two places, one of which is the Moconá Falls.
The falls itself are not visible for 150 days a year when the river is full. During this period, the falls become more like rapids. When water level becomes low and falls below the edge of the canyon, it starts spilling into the now exposed canyon, and the Mocona Falls is formed. Depending on the volume of the water dragged by the Uruguay River, the height of the falls varies from five to seven meters. The width of the waterfall is also subjected to water volume ranging between 1,800 meters and 3,000 meters wide.
Moconá Falls and the provincial park surrounding them are a popular tourist destination. See more pictures, and read about the falls, at Amusing Planet. -via the Presurfer
(Image credit: Flickr user Viajá por tu País)