Running water present in almost all environments, at least occasionally but other agents are limited to small parts of the continents (action of wind, waves and currents) or glacial ice.
Nearly every part of the Earth has seen, at sometime in it’s geologic past, the imprint of fluvial processes.
In this paper we will discussed about the various types of erosion, features of drainage basin and types of drainage pattern geomorphic works of rivers: erosion, transportation and deposition, the development of river valley, the stages of river and various land forms associated with these courses.
River is anybody of fresh water flowing from an upland source to a large lake or to the sea, fed by such sources as springs and tributary streams.
A river starts on hillsides as small channels, or rills.
The rills combine to make larger channels or tributaries that eventually come together, forming distinct streams.
The largest channels formed by this convergence of tributaries are rivers, and they can carry large quantities of fresh water and sediment across continents.
The world’s longest rivers Cont’d River name Continent Total length Nile Africa 6695 Km Amazon South America 6400 Km Yangtze Asia 6300 Km Mississippi-Missouri-Red Rock North America 5970 Km Yeisey-Angara Asia 5550 Km Yellow River Asia 5464 Km Ob’-Irtysh Asia 5410 Km Rio Parana-ril grande South America 4500 Km Amur-shilka Asia 4416 Km Lena Asia 4400 Km Congo Africa 4374 Km Mackenzie-Peace-Finlay North America 4241 Km Mekong Asia 4200 Km Niger Africa 4180 Km
They may start from the melt waters of glaciers, e.g. Rhone (France), lakes, e.g. Nile (Africa), springs, e.g. Thames (England) from regions of steady rainfall, e.g. Zaire (Africa).
Likewise, they may end at in the sea, e.g. Amazon (Atlantic), the Niger (Gulf of Guinea) and the Indus, (Arabian Sea). Sometimes the mouth could be in a lake, e.g. Volga (Caspian), or in a swamp, e.g. Chari River (Lake Chad).
Moves from its sources to its mouth captures several tributaries.
Geologists have devoted much time and effort to the study of drainage systems and their evolution. The reasons for their interests are:
Firstly, there is the obvious point that a drainage system is a major feature of the physical landscape.
Secondly, evolutionary studies of drainage systems may afford valuable information about the denudational history of an area. For instance, it is often useful to attempt a reconstruction of the initial form of a river system in order to gain evidences of the nature and mode of origin of the land surface on which that system began its existence.
Streams within the drainage basin are either perennial or intermittent in flow.
Perennial streams -permanent streams that flow all year.
Intermittent stream — a stream that carries water only part of the year.
Tributary streams- small streams that enter into the main stream. Some streams are classified as exotic streams , originate in a humid region but flow through an arid region. E.g.. Nile and Colorado Rivers
Superposed Streams Development of water gap Folded Units Cont’d
With in a stream channel, three types of flow can be observed
Laminar flow -Under very low velocities water flows through a stream as smooth sheets running parallel to the bed In this type of flow the direction of water in the stream is not altered in its direction
Only the finest particles kind be detached, so laminar flow is basically non erosive.
Turbulent flow - under higher flow velocities, resistance within the flow and that caused by the bed and sides of the channel (channel topography) cause the flow to break down into separate currents.
- More erosive than laminar flow and help suspend material in the stream.
- Is the "normal" type of flow in most streams
Helical flow - spiral flow in a stream, Caused by channel shape.
In streams in humid climates, discharge increases downstream for two reasons:
as river water evaporates into the air
and soaks into the dry ground
In an arid climate, a river’s discharge can decrease in a downstream direction
1. Water flows out of the ground into the river through the streambed. 2. Small tributary streams flow into a larger stream along its length, adding water to the stream as it travels. Discharge - is the volume of water that flows past a given point in a unit of time.
Saltation :t he speed of the water lifts the fragments( tiny pebbles ) of the river bottom, and they bounce along and leapfrog over each other in a process called saltation (from a Latin word meaning jump).
- Saltation, therefore is a combination of traction and suspension.
Energy within the system is not evenly distributed along the profile; contains falls and basins.
Falls result in a concentration of energy, which promotes erosion.
Basins result in a decrease in energy, which promotes deposition.
Young Stage -Has steep-sided and narrow V-shaped valley created by vertical erosion -characterized by waterfalls and rapids. -Water is often quite clear because the river is not carrying much load in suspension. -Velocity is high. -Down cutting and head ward erosion are the major processes of river erosion. and head ward erosion is dominant
4. Gradually, the inside banks are filled in with accumulated deposits, and the outside bends extend further and further, forming a wide loop in the river.
5. The loop continues to bend further and further, until a thin strip of land called a neck is created at the beginning and the end of the meander
6. Eventually, the narrow neck is cut through by either gradual erosion or during a time of flooding. When this happens, a new straighter channel is created, diverting the flow of the river from the loop into the new channel.
7. Deposition finally seals the cut-off from the river channel, leaving a crescent-shaped land form known as oxbow lake.
Meander scar - feature left behind when the water in an ox-bow lake dries up
When the river floods it breaks through the thin meander neck and the river takes the easier, straight course. This leaves the meander loop ‘cut off’ as an oxbow lake. Over time, the oxbow lake will become colonised by vegetation.
Final stage of a river is reached when the river is flowing on more or less flat surface.
The speed so low that no net erosion occurs any more.
Erosion on the slopes is balanced by deposition on the floodplains.
Peneplain -a flat and relatively featureless landscape with minimal relief; considered to be the end product of the geomorphic cycle. Peneplain means “almost a plain. The process is known as peneplanation
From now on the rivers just sweep over the floodplain and rework the sediment, creating various floodplain features. Isolated remnants of resistant bedrock may rise over the Peneplain surface, the so-called monad nocks or inselbergs .
Yazoo stream _ a tributary unable to enter the main stream because of natural levees along the main stream; instead flows downstream to the back swamp zone and runs parallel to the main stream until it finds an entrance.
is a narrow ridge of alluvium deposited at the side of the channel. During high discharge periods when the stream floods, coarse sediment settles out near the stream channel and grades to finer material further away.