The Life Of A River Most rivers are first started when rainfall collects on the ground. If the collected water is on any kind of a slope then the water will travel down the slope and form into individual channels.
As well as rainfall these channels can be fed by underground springs. The movement of this water will start to erode the valley. As these channels grow they erode the ground away to form gullies, then brooks, then creeks, and someday a large stream or river. The small streams at a rivers source that unite to form a main channel are called headwaters. Sometimes a headwater, which can also be a lake or a marsh, may overflow on its lowest edge.
This will eventually form a new stream. When a river empties into another body of water it is called the mouth of a river. When there are smaller streams that flow into a larger stream these are called tributaries. When there are many tributaries connecting into a river they are called a river system and the area drained by a river system is called the drainage basins. Most of the time there is a common drainage pattern that rivers follow.
That pattern is the compared to the drainage pattern of a leaf. Drainage basins are divided by elevated areas of land which are known as watersheds or divides. Now that all of that is covered there are three major stages of a rivers development. The stages are: Youth, Mature, and Old Age.
After a river has formed and established a definite channel it will start to deepen, widen, and lengthen its valley. A youthful river will travel down a relatively steep gradient or slope. While traveling down the stream the river uses most of its energy to deepen its valley. Being a young river greatly increases their capacity to erode materials and carry them down river. Most of the small materials are suspended in the water as they are carried away. As well as eroding rocks and other materials the river picks up minerals which is dissolved into the water.
When a river erodes large materials that are too heavy to be suspended in the water they are rolled and bounced along the riverbed. As a youthful river continues its erosion process will create a narrow steep-sided valley. Young rivers commonly contain lots of rapids and also waterfalls. These are created by the parent rock, which is the bedrock, which change types as you move down the river. The difference in the bedrock allows the river to erode away the rock that is less resistant to erosion while leaving the rock that is resistant to erosion.
By the time a river becomes mature it has cut down its gradient so much that the river is more gentle and it starts to turn more. By now it has cut away most of the rapids and the waterfalls. Also the rivers speed and eroding capabilities has greatly reduced. Now instead of cutting the valley down it starts to work to cut the valley wider. Mature rivers erode the valley walls by laterally spreading. Because the river is now starting to turn the sediment is starting to slam from side to side as the river turns.
This action of eroding uncovers the valley floor which in turn creates more of a meandering motion. The uncovered floor becomes prone to flooding, so it is considered a floodplain. A floodplain is built of sediments deposited by the river that flows through it and is covered by water during floods when the river overflows its banks. During most floods, just a portion of the floodplain is covered with water and only during infrequent, very large floods is the whole floodplain covered. Floodplains tend to develop on the lower and less steep sections of rivers.
River channels in floodplains adopt two kinds of patterns: meandering and braided. Meandering rivers consist of a single main channel that bends and loops. In some cases, the channel is so winding that the length along the channel is several times the straight-line distance along the river valley. Braided rivers have numerous distinct channels that repeatedly divide and then merge again downstream.
While a meandering channel occupies only a small part of its floodplain at any one time, a braided river occupies much of the floodplain over the course of a year. Both patterns migrate across the floodplain, removing sediments from their path and depositing them elsewhere. A braided river reworks the sediment in its floodplain very frequently as the various individual channels continually shift position.Meandering channels are more common where the floodplain sediments are sand, silt, and clay. Braided channels are more common where the floodplain sediment is mostly gravel or where there is an increase in channel steepness. A braided pattern also tends to be favored if the amount of water flowing in the river is highly variable or if the turns are easily eroded and can provide abundant sediment to the channel.
In meandering rivers, sediment is eroded on the outside of turns and deposited on the inside of turns. Over time, this causes meander loops to migrate downstream.If the movement of one meander loop overruns the next one downstream, then a meander cut-off, or chute, is formed. This causes the course of the channel to be shortened as the two meander loops join. The abandoned meander loop is gradually isolated as sediment is deposited at each end by the water flow in the main channel.
This process eventually leads to the creation of an oxbow lake. When a river becomes an old age river it has now lost most of its ability to erode away the valley walls and also its speed has been greatly reduced. The sediments that the river has been collecting all its life is now being deposited onto the riverbed. Rivers can be simultaneously young mature and old. If a river shows any combination of stages it is obviously like this.Once a river starts to begins its drainage basin it starts to deposit the majority of it sediments. This depositing starts to create a delta.
The materials in the delta can be called an alluvium. An alluvium consists of silt, sand, and pebbles. A delta starts to form when a river enters an ocean, a lake, or any type of body of water that is slower than it is. This will create the river to slow which in turn creates the bouncing or suspended materials to sink to the bottom. The sediments are continuously dumped until the sediments build up above the river. Most deltas take the shape of a triangle.
The size of the delta is determined by several different factors. The first being the speed of the river.If the river is slow the sediments wont be pushed out past the mouth of the river and will build up. If the river is too fast the sediments will be pushed too far from the mouth of the river and the delta will be unable to collect the sediments and it wont be built. The second is the tidal action at the mouth of the river. If the waves near the mouth are too heavy the formation of the delta may be limited and may even be prevented from forming if the waves are severe enough. The last factor that could determine the size of the delta is the composition of the delta.
This means that depending on the content of the sediment in the river the delta will be unable to form. Once a river is able and has built a delta the water from the river sometimes flows over the banks and creates a new stream called a distributary. If the river floods the newly formed distributary may suddenly change shape and change channels. The soil of a delta is usually rich and fertile.
In some places this fertile soil is used for agricultural purposes.
