1.12 - Introduction to River Landscapes
The concept of a river's course and its sections
A river's journey from its starting point to where it meets the sea is known as its course. This path is divided into three distinct sections: the upper course, middle course, and lower course. Each section displays unique features due to the varying influences of natural processes as the river flows downhill.
Defining the sections of a river's course
- Upper course - Located nearest to the river's source, often in elevated areas, where the river begins its descent.
- Middle course - The central section of the river, transitioning between steep and flat terrains.
- Lower course - The final stretch, close to the river's mouth, where it approaches its endpoint, often a sea or lake.
Rivers carve out channels and valleys as they travel, eroding the landscape, carrying materials downstream, and depositing them along the way. The dominant process—whether erosion or deposition—greatly influences the shape of the valley and channel in each section.
The long and cross profiles of a river
The shape and steepness of a river's path can be understood through two key perspectives: the long profile and the cross profile.
Understanding river profiles
- Long profile - This illustrates the gradient, or steepness, of the river from source to mouth. It shows a steep decline in the upper course, a more moderate slope in the middle course, and a near-flat gradient in the lower course.
- Cross profile - This represents a cross-section of the river valley and channel at a specific point, highlighting variations in width, depth, and valley shape across the different courses.
Characteristics of the upper, middle, and lower courses
Each section of a river's course exhibits distinct physical features influenced by the landscape, gradient, and dominant processes.
| River course | Gradient and valley shape | Channel characteristics | Landscape and rock type | Sediment load |
|---|---|---|---|---|
| Upper course | Steep gradient with a V-shaped valley due to intense vertical erosion, resulting in steep-sided valleys | Narrow and shallow channel with a rough bed and sides causing high friction, leading to lower velocity and discharge | Often found in high elevation areas with hard, resistant rock that resists erosion | Carries large, angular stones due to limited erosion and transport time |
| Middle course | Medium gradient with gently sloping valley sides, widened by lateral erosion | Wider and deeper channel compared to the upper course, with smoother sides reducing friction and increasing flow speed and discharge as more streams join | Flows through areas with softer, less resistant rock, which erodes more easily | Material becomes smaller and more rounded due to ongoing erosion processes |
| Lower course | Gentle gradient with a very wide, almost flat valley due to extensive lateral erosion | Very wide and deep channel with smooth sides, resulting in minimal friction, high velocity, and large discharge from tributary inputs | Typically flows through flat, low-lying areas with easily erodible materials | Carries fine, well-rounded particles, mostly transported by suspension or solution |
Processes of erosion shaping river valleys and channels
Erosion plays a critical role in sculpting the river's path, with two main types—vertical and lateral—affecting the valley and channel differently across the river's course.
Types of erosion in river formation
- Vertical erosion - Predominant in the upper course, this process deepens the river valley and channel, creating the characteristic V-shape. High turbulence causes rough, angular particles to scrape along the river bed, leading to intense downward erosion.
- Lateral erosion - More significant in the middle and lower courses, this process widens the river valley and channel, especially during the formation of meanders. It erodes the sides of the river, reshaping the landscape over time.
The concept of a river basin and sediment load changes
A river's influence extends beyond its immediate channel, affecting a broader area and transforming the materials it carries as it progresses downstream.
Understanding river basins and sediment dynamics
- River basin - This is the surrounding land area where any rainfall eventually drains into the river. It acts as a catchment zone, feeding water into the river system through streams and tributaries.
- Changes in sediment load - As the river moves from the upper to the lower course, the sediment it carries becomes smaller and more rounded. This transformation occurs due to continuous erosion and abrasion during transport, smoothing and reducing the size of particles over time.