Flow obstructions may be caused by ice, may inhibit drainage or cause erosion, and may ultimately lead to flooding. They may also be unrelated to these conditions, constituting a hazard in their own right to such activities as hydropower generation and navigation. They may even cause the displacement of river channels. Obstructions to the flow of a watercourse may result from human activity, but are often caused by events such as landslides and earthquakes, by natural logjams, or by such processes as the accumulation of sediment or of debris.