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Here's the answer to the often asked question 'how are rivers formed'. A river is a natural stream of water, usually freshwater, flowing toward an ocean, a lake, or another stream. In some cases a river flows into the ground or dries up completely before reaching another body of water. Usually larger streams are called rivers while smaller streams are called creeks, brooks, rivulets, rills, and many other terms, but there is no general rule that defines what can be called a river. Sometimes a river is said to be larger than a creek, but this is not always the case.
But this hasn't yet answered 'how are rivers formed'. A river is a component of the water cycle. The water within a river is generally collected from precipitation through surface runoff, groundwater recharge (as seen at baseflow conditions / during periods of lack of precipitation) and release of stored water in natural reservoirs, such as a glacier.
A river may have its source in a spring, lake, from damp, boggy landscapes where the soil is waterlogged, from glacial melt, or from surface runoff of precipitation. Almost all rivers are joined by other rivers and streams termed tributaries, the highest of which are known as headwaters. Water may also originate from groundwater sources. Throughout the course of the river, the total volume transported downstream will often be a combination of the free water flow together with a substantial contribution flowing through sub-surface rocks and gravels that underlie the river and its floodplain. For many rivers in large valleys, this unseen component of flow may greatly exceed the visible flow.
So, now that we know more about how are rivers formed, what else is there to know? From their source, rivers flow downhill, typically terminating in a sea or in a lake, through a confluence. In arid areas rivers sometimes end by losing water to evaporation. River water may also infiltrate into the soil or pervious rock, where it becomes groundwater. Excessive abstraction of water for use in industry, irrigation, etc., can also cause a river to dry before reaching its natural terminus.
The mouth, or lower end, of a river is known as its base level. The area drained by a river and its canals is called catchment, catchment basin, drainage basin or watershed. The term "watershed" is also used to mean a boundary between catchments, which is also called a water divide, or in some, continental divide.
The water in a river is usually confined to a channel, made up of a stream bed between banks. In larger rivers there is also a wider floodplain shaped by flood-waters over-topping the channel. Flood plains may be very wide in relation to the size of the river channel. This distinction between river channel and floodplain can be blurred especially in urban areas where the floodplain of a river channel can become greatly developed by housing and industry.
The term upriver, is referred to as the beginning or source of the river flow regardless of the direction of flow. Therefore, the term down river, is referring to the direction of flow that the river continues in.
The river channel typically contains a single stream of water, but some rivers flow as several interconnecting streams of water, producing a braided river. Extensive braided rivers are found in only a few regions worldwide, such as the South Island of New Zealand. They also occur on peneplains and some of the larger river deltas. Anastamosing rivers are similar to braided rivers and are also quite rare. They have multiple sinuous channels carrying large volumes of sediment.
There you have it, the answer to 'how are rivers formed'. A river flowing in its channel is a source of energy which acts on the river channel to change its shape and form. According to Brahm's law (sometimes called Airy's law), the mass of objects that may be flown away by a river is proportional to the sixth power of the river flow speed. Thus, when the speed of flow increases two times, it can transport 64 times larger (i.e. more massive) objects.[3] In mountainous torrential zones this can be seen as erosion channels through hard rocks and the creation of sands and gravels from the destruction of larger rocks. In U shaped glaciated valleys, the subsequent river valley can often easily be identified by the V shaped channel that it has carved. In the middle reaches where the river may flow over flatter land, meanders may form through erosion of the river banks and deposition on the inside of bends. Sometimes the river will cut off a loop, shortening the channel and forming an oxbow lake or billabong. Rivers that carry large amounts of sediment may develop conspicuous deltas at their mouths, if conditions permit. Rivers whose mouths are in saline tidal waters may form estuaries.
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