Rivers and estuaries play an important role in the water cycle. A river is a natural source of water that flows towards oceans or lakes. Smaller rivers are known as streams or creeks and usually adjoin to larger rivers. Rivers, creeks and streams may form from precipitation or runoff from areas of higher elevation. Rivers may also collect water that is released by ice or glaciers, as well as from springs and other still water sources. Rivers usually flow in the same direction (towards a source such as an ocean), which creates banks on either side of the water. Estuaries are coastal bodies of water that open up to oceans, and are usually met by one or more rivers flowing into them. Estuaries are influenced by the activity of both rivers and oceans, resulting in an abundance of life forms living in them.
Rivers and estuaries have a profound effect on the everyday lives of humans and animals alike. For example, humans are able to manage rivers for both protection and beneficial purposes. Dams may be built in order to store water and conserve energy, and the risk of flooding may be prevented through the construction of dikes. These water sources also support a wide variety of life that may be beneficial in supporting the human lifestyle; for example, extracted river water can then be purified and stored for drinking water. The natural flow of water in rivers and estuaries may also be utilised to provide hydropower (energy provided by the force of water) for factories and other industrial practises.
Rivers and estuaries may also become problematic. Erosion of sediment on river banks may cause flooding or even pollution due to debris entering the river. Runoff from agricultural operations, factories, and sewers may contaminate water in rivers, and eventually estuaries and the ocean. Because many sewage operations empty into rivers or oceans, problems with pollution are not uncommon. Health issues and deaths related to the consumption of contaminated water have become one of the leading problems throughout the world. In many areas of the world, individuals are unable to enjoy the benefit of having purification systems for dirty or contaminated water. In poor countries, many people obtain water directly from rivers and use this water for drinking, cooking and bathing. The threat of contamination is significantly higher in areas where economic situations are unable to manage or prevent the threat of pollution in water sources, such as rivers and estuaries.