What feature is created when a river flows over a band of hard rock, resulting in a cascade of water?

A waterfall is formed when a river encounters a band of hard rock, leading to a sudden drop in elevation. This geological process creates a dramatic and picturesque cascade of water, as the softer rock upstream often erodes more quickly compared to the hard rock layer. As the water flows over the edge of this hard rock, it falls into a pool or the riverbed below, resulting in the characteristic vertical drop associated with waterfalls. They are significant features in a river's course and are often indicators of changes in the geology of the riverbed. The presence of a waterfall can also lead to areas of turbulent water both upstream and downstream, which contributes to the dynamic nature of river environments.

In contrast, features such as rapids involve sections where water flows quickly over rocky areas, but they do not necessarily involve a vertical drop like a waterfall does. Deltas represent the deposit of sediment at the mouth of a river where it meets a larger body of water, and lakes are typically standing bodies of water. Each of these features is distinct in its formation and characteristics compared to a waterfall.