Waterfalls are commonly formed in the upper course of a river in steep mountains. Because of their landscape position, many waterfalls occur over bedrock fed by little contributing area, so they are ephemeral and only occur during rainstorms. The further downstream, the more perennial a waterfall can be. It is even possible for waterfalls to occur far downstream due to transient discontinuities in a river's longitudinal profile caused by climatic changes and anthropogenic activities. Waterfalls can have a wide range of widths and depths, and this diversity is part of what makes them such a charismatic and interesting natural phenomenon. Research into the diversity of natural waterfalls systematics has been carried out in recent years.
