Roughly 97% of the Earth’s water is found in our oceans. The other 3% is fresh water. Most of this fresh water is locked in glaciers and pack ice or is located underground. Only about 0.3 % of the Earth’s water is found in rivers and lakes. Nonetheless, rivers play a very important role in the water cycle and also in our lives.
A river is a fairly large body of water which flows in a channel and through which excess water is drained away from the land. The place where a river begins is called its source. A river’s source may be a spring, a lake, or even a glacier. The place where a river ends is called its mouth. A river may end its journey in a sea, an ocean or a lake.
A river which flows into another river is known as a tributary. The place where a tributary joins a larger river is called a confluence. A main river and all its tributaries is called a drainage system. All the land drained by a river and its tributaries is known as a drainage basin. The high land which separates one drainage basin from another is called a watershed or divide.
A river and its tributaries often from a recognizable pattern known as a drainage pattern. The CXC/CSEC syllabus requires students to study dendritic, radial and trellis drainage patterns.
The dendritic drainage pattern (seen below), is the most common.
The word dendritic comes from the Greek word for tree (dendron). True to its name, this drainage pattern resembles a tree. The main river looks like the trunk of a tree, the tributaries of the main river resemble the branches and the small streams are like twigs. Dendritic drainage patterns develop in areas where the rock type is uniform and the land is gently sloping. In other words, this pattern is likely to develop wherever there are no major geological factors influencing drainage patterns.
A radial drainage pattern occurs when rivers flow away from a central area of high land. Excellent examples of radial drainage patterns occur where rivers flow down the slopes of volcanoes or domes (see diagram below).
A good example of a radial drainage pattern is found in Montserrat, where rivers flow away from the Soufriere Hills volcanic centre (refer to map below).
Trellis drainage patterns occur when tributaries join a main river at right angles. This pattern often occurs in areas which have alternating bands of resistant and less resistant rock at right angles to the direction of the slope. The main river flows across the bands of rock, following the slope of the land. Tributaries follow the bands of less resistant rock and join the main river at right angles.
The Work of Rivers
Rivers are a powerful force shaping the landscape. They perform three basic types of work; erosion, transportation and deposition.
How does a river erode?
Erosion refers to the removal of weathered material by agents such as wind, water and ice. Running water is one of the most efficient agents of erosion and one of the most powerful. A river erodes in many ways:
Hydraulic action: This refers to the force of the water itself. Water splashes against river banks and forces it way into cracks and openings. This loosens material along the bed and banks and the river carries this material away.
Solution: Some minerals in rocks and soil are soluble. They are slowly dissolved by river water as it comes into contact with them. Examples of such minerals are salt and calcium carbonate.
Corrasion/ abrasion: As the river flows it grinds its load against its bed and banks. This loosens material which the river carries downstream.
Attrition: This refers to the process by which material being carried by the river, such as rocks and stones, collide with each other. Over time, they become smaller and smaller. They also become smooth and rounded.
Watch the video about river erosion below.
How do rivers transport material?
The term transportation refers to the ways in which a river carries material. The material that a river carries is called the river’s load. A river carries its load in many ways:
Solution: Materials which are dissolved in the water are carried downstream.
Suspension: Tiny particles such as silt are suspended in the water as the river flows. During periods of heavy rainfall, the water in rivers often turns brown because of all the material suspended in it.
Saltation: Particles such as small stones are bounced along the riverbed as they make their way downstream.
Traction: The heaviest material in the river’s load, such as large rocks, are simply rolled along the riverbed.
Check out this short video on transportation by rivers!
Deposition by rivers
Deposition occurs when a river drops all or part of its load. It occurs when a river does not have enough energy to continue carrying its load either because the river slows down or the amount of water in the river’s channel decreases. As a river begins to lose energy, the heavier material is deposited first while the lighter material is carried farther.