As a river flows downstream, sediment undergoes a noticeable transformation, primarily becoming smaller, finer, and smoother.
Downstream Sediment Transformation
A number of river characteristics change as you travel downstream, and one of the most significant is the nature of the sediment carried by the water. Based on river processes, the sediment load evolves in several key ways:
Decrease in Sediment Size
Firstly, sediment size decreases as you travel downstream. This reduction in size is a direct result of ongoing erosion processes acting on the sediment particles.
Change in Sediment Texture
Accompanying the decrease in size, sediment also becomes finer and smoother. This is because the rough edges and larger fragments are gradually broken down and worn away.
The Role of Erosion
This transformation happens due to erosion. Erosion is the process by which natural forces wear away rock and soil. Throughout the course of the river, various types of erosion constantly act upon the sediment particles.
The specific processes that cause this sediment change include:
- Hydraulic Action: The force of the water itself, compressing air in cracks and breaking off pieces.
- Abrasion: The grinding and wearing away of the river bed and banks by the sediment particles carried by the water.
- Attrition: The wearing down and breaking of sediment particles as they collide with each other.
- Solution: The dissolving of soluble minerals from the rock and sediment by the water.
These processes collectively break down larger, angular sediment found upstream into smaller, more rounded particles that are characteristic of downstream sections of a river.
Upstream vs. Downstream Sediment
Here is a simple comparison of sediment characteristics typically found in the upper versus lower courses of a river:
| Characteristic | Upstream (Upper Course) | Downstream (Lower Course) |
|---|---|---|
| Sediment Size | Larger, heavier | Smaller, lighter |
| Sediment Shape | More angular, rougher | More rounded, smoother |
| Dominant Transport | Rolling, bouncing (Bedload) | Suspension (Suspended load) |
Understanding how sediment changes downstream is crucial for studying river geomorphology and the dynamic processes shaping river landscapes.
