River shape and size dramatically changes with the presence of a dam. Because of a dam there is a restriction of water from its natural flow and will change the flow rates due to water restrictions. (See Flow Rates) By releasing large amounts of water throughout the day (especially in hydroelectric dams) it flushes the riverbed and strips away organic materials, sediment, vegetation, and macro-invertebrates. Unnatural changes in flow erode banks and heavily damages the riparian zone of rivers. The constant "bobbing" of water levels creates an unstable river bank that looks similar to "bathtub rings". Two-thirds of rivers around the world have been affected by dams. Many large influential rivers including: the Indus, the Colorado, and the Yellow Rivers no longer reach the ocean. This has turned the rivers thriving size and wildlife into a biological desert.