Waterfalls walk backwards, carving gorges as they retreat upstream
A waterfall is not a fixed feature but a slow-motion retreat. Spinning stones drill potholes, splashback hollows out soft rock beneath the lip, and eventually the overhanging cap collapses. Some falls pull back upstream as fast as one and a half metres a year, leaving a canyon behind them.
The classic recipe, known as the caprock model, starts with a river flowing across a hard layer of bedrock that sits on softer rock. The soft layer wears away faster, so the drop grows steeper. At the base, hydraulic jets and churning sediment hammer the riverbed, and horseshoe-shaped falls concentrate that energy on a single spot. Undercutting often leaves a shallow cave behind the curtain, and fallen blocks grind one another down, deepening the plunge pool.
Other routes exist. Faults can create a sudden step in a river's course. Glaciers carve deep troughs whose tributaries are left hanging high above the main valley once the ice melts; Yosemite's great falls are hanging valleys of this kind. Earthquakes, landslides and volcanoes can create falls abruptly, and in the ocean, cold water plunging over a gorge where it meets warmer water can produce enormous underwater cascades.
Defining a waterfall is trickier than it sounds. A common threshold is five feet of height, though some sources demand twenty; Mill Pond Falls in Newington, Connecticut, at 12 feet, claims to be the smallest natural fall in the United States. In 2000 the researcher Mabin proposed that the lip should sit no further forward of the plunge pool than a quarter of the fall's height. Cataract usually suggests a big, powerful fall and cascade a smaller one, while northern English dialect offers foss or force.
Falls shape life around them. They block fish and aquatic insects from moving upstream, so communities above and below can differ sharply, and specialised plants and insects live in the spray zone itself. Their cool, humid microclimates support distinctive land species too; a 2012 study of the Agbokim Waterfalls found far more biodiversity than expected. People have sought them out since prehistory as sacred sites, and since the 18th century as tourist attractions and sources of hydropower.
Source: Waterfall
