Rivers: Drainage Basins and Long Profiles
Rivers play a crucial role in shaping the Earth's surface through various river erosion processes and landforms. This section introduces key concepts related to river systems and their characteristics.
A drainage basin is defined as an area of land drained by a river and its tributaries. This concept is fundamental to understanding the scope and impact of river systems on the surrounding landscape.
The long profile of a river changes as it flows from its source to its mouth. In the upper course, the profile is typically steep, while in the lower course, it becomes more gentle. This change in gradient affects the river's erosive power and the types of landforms it creates along its course.
Definition: A drainage basin is an area of land drained by a river and its tributaries.
Highlight: The long profile of a river changes from steep in the upper course to gentle in the lower course, influencing erosion and deposition patterns.
Erosion Processes
River erosion is a complex process that occurs both vertically (downwards) and laterally (sideways). There are 4 types of river erosion that contribute to changing the river channel and valley:
- Hydraulic action: The force of water hitting the riverbed and banks, most effective when water is moving fast at a high volume.
- Abrasion: The load carried by the river hits beds or banks, dislodging particles.
- Attrition: Stones carried by the river knock against each other, becoming smaller and more rounded.
- Solution: Alkaline rocks are dissolved by slightly acidic river water.
Vocabulary: Hydraulic action, abrasion, attrition, and solution are the four main processes of river erosion.
Transportation Processes
The material transported by a river is called its load. There are 4 processes of fluvial transportation:
- Solution: Dissolved load
- Suspension: Small sediment held in the river
- Traction: Large particles rolled on the river bed
- Saltation: Bouncing of particles too heavy to be suspended
The size and amount of load carried depends on the river's speed or velocity.
Example: Saltation occurs when particles are too heavy to be suspended but can be bounced along the riverbed by the force of the water.
River Erosion Landforms
Several distinctive landforms are created through river erosion processes:
- Interlocking spurs: Formed in mountain streams as the river erodes vertically, creating a V-shaped valley and winding around areas of resistant rock.
- Waterfalls: Develop where a river flows over different rock types, with more resistant rocks forming steps in the river's long profile.
- Gorges: Steep-sided, narrow valleys found downstream of retreating waterfalls.
Highlight: Waterfalls can also form in glacial hanging valleys or when sea levels drop, causing a river to cut down into its bed.
Changes in Cross Profile
The cross profile of a river changes along its course due to the amount of water flowing in the river. As tributaries add more water, the river erodes its channel, making it wider and deeper. The cross profile evolves from:
- V-shaped valley: Steep-sided, narrow, shallow, and turbulent river
- Floodplain: Wider valley with a flat floor, wider and deeper river
- Levees: Very wide and flat valley, wide and deep river with a large sediment load
Definition: The cross profile is the shape of the valley from one side across to the other.
Deposition Processes
Deposition occurs when a river's velocity decreases, and it no longer has the energy to transport its load. The process of deposition varies along the river's course:
- Larger rocks are deposited in the upper course during periods of high flow.
- Smaller sediment is carried further downstream and deposited on the river's bed and banks where velocity slows down due to friction.
- Significant deposition occurs at the river's mouth due to the gentle gradient and interaction with tides.
Highlight: The size of sediment deposited is directly related to the river's velocity and energy.
