A waterfall is an area where water flows over a vertical drop or a series of steep drops in the course of a stream or river. Waterfalls also occur where meltwater drops over the edge of a tabular iceberg or ice shelf. Waterfalls are commonly formed in the upper course of a river where lakes fall into in steep mountains. Because of their landscape position, many waterfalls occur over bedrock fed by little contributing area, so they may be ephemeral and flow only during rainstorms or significant snowmelt