A waterfall is a point in a river or stream where water flows over a vertical drop, often created by a river crossing from a layer of resistant rock to a softer one. Waterfalls are dynamic features that are constantly subject to change. Their cessation or transformation can occur rapidly due to short-term climatic events, slowly over geologic timescales through erosion, or immediately through human intervention.
Seasonal and Climatic Factors
Short-term climate shifts are the most common reason a waterfall might cease to flow, though this change is almost always temporary and reversible. A prolonged period of severe drought can significantly reduce the water volume in the river upstream, causing the flow over the falls to slow to a trickle or stop completely. This is driven by a lack of precipitation or snowmelt feeding the river system.
In extremely cold climates, a waterfall’s flow can be halted by freezing temperatures. Extended periods below the freezing point can lead to supercooling, encouraging the formation of frazil ice—tiny particles that adhere to cold rocks and cliff faces. As water flows over this accumulation, the ice builds up and completely covers the falls, creating a temporary ice dam that stops the flow until temperatures rise.
Geological Retreat and Erosion
The long-term, irreversible fate of most waterfalls is determined by headward erosion, or headward retreat. This natural process causes the waterfall to physically move upstream, which can take thousands of years. The geological structure necessary for a waterfall consists of a hard, erosion-resistant caprock layer overlying softer rock layers like shale or sandstone.
The force of the falling water carves out a deep basin at the base of the falls known as a plunge pool. The turbulence within this pool, combined with the abrasive action of trapped sediment, undercuts the softer rock beneath the hard caprock. As the softer rock is eroded away, the resistant caprock overhangs the plunge pool until its own weight causes it to collapse into the basin. This collapse shifts the vertical drop slightly upstream, and the cycle immediately begins again.
The process continues until the elevation difference is eliminated, or the falls reach a section of the riverbed composed entirely of resistant rock, eventually transforming the sheer drop into a series of rapids or a smooth slope. For example, the rate of retreat at Niagara Falls has been estimated at about one meter per year under normal flow conditions.
Anthropogenic Changes to Water Flow
Human activities represent the most rapid and permanent way a waterfall can be silenced, often without any change to the physical rock structure itself. This is primarily achieved by manipulating the river’s hydrology upstream of the falls. The construction of large-scale infrastructure, such as dams and reservoirs, diverts the river’s volume for uses like hydroelectric power generation, irrigation, or municipal water supply.
In these cases, the water that would naturally flow over the waterfall is channeled through pipes or tunnels, leaving the original channel dry or significantly reduced. For instance, a substantial portion of the Niagara River’s flow is routinely diverted through tunnels to power hydroelectric plants, especially outside of peak tourist hours. The complete submersion of the once-powerful Guaíra Falls in 1982 due to the creation of a dam is another example where human engineering permanently ended a waterfall’s existence.