Waterfalls are striking geographical features that mark a sudden drop in a river’s longitudinal profile, interrupting the natural, gradual slope of the watercourse. These dramatic breaks in the landscape necessitate specific terminology to describe their anatomy, particularly the point where the horizontal flow of the river transitions into a vertical plunge. The starting point of this descent, the very edge the water flows over, is a highly specific and actively eroded part of the river system.
The Specific Names for the Waterfall’s Edge
The most accurate and widely used term for the top of a waterfall, the horizontal line where the water begins its drop, is the crest. The crest is a geological reference point marking the boundary between the upstream river channel and the vertical fall face. This term is often used interchangeably with the brink in most geographical and hydrological contexts. Both the crest and the brink define the uppermost line where the flowing water separates from the bedrock surface.
A less formal term is the lip, which also refers to this projecting edge over which the sheet of water flows. This line is where the river’s flow transforms from a relatively slow state to a rapid, supercritical plunge.
Physical Components Surrounding the Crest
Immediately preceding the crest is the approach channel, the section of the riverbed leading directly up to the drop-off point. Water flowing through the approach channel accelerates as it nears the edge, often exhibiting a downward curve in the water surface called the drawdown.
The lowest point along the crest line, where the bulk of the water flows over, can sometimes be referred to as a notch. This notch dictates the primary path and concentration of the water’s erosive power. Once the water leaves the crest, the vertical rock face it falls down is known as the fall face. The sheet of water itself, as it springs free from the crest, is hydrologically termed the nappe.
Understanding Crest Erosion and Retreat
The location and shape of the crest are constantly modified by geological processes. Waterfalls are often maintained by differential erosion, where a layer of hard, resistant rock (caprock) overlies softer, less resistant rock layers. The caprock forms the protective surface of the crest, but the softer rock beneath it is eroded more quickly by undercutting.
Undercutting occurs as the falling water and the turbulent water in the plunge pool at the base erode the softer material behind the fall face. This action creates a shallow cave-like formation beneath the caprock overhang. When the overhang becomes unstable and collapses, the crest of the waterfall moves backward, a process known as headward erosion or retreat. Niagara Falls, for instance, has retreated approximately 11 kilometers over thousands of years due to this continuous process. Waterfalls are temporary interruptions in a river’s profile that slowly migrate upstream until they are eventually eliminated.