River Systems: Fluvial Processes and Geomorphological Evolution

The River System

Much of the rainwater that falls on the Earth's surface is concentrated in flows or channeled river currents.

Fluvial Processes: Load, Capacity, and Competence

The action of a river (erosion, transport, and sedimentation) for a given speed depends on the size of the particles. The action of a river is defined by:

• Load (Freight): The actual amount of sediment a river carries at a certain time and place. This may include:
  • Bed Load: Larger particles carried by rolling (pebbles and gravel) or saltation (sand).
  • Suspended Load: Finest materials (clay and silt) carried within the water column.
  • Dissolved Load (Transport in Solution): Materials transported as soluble compounds (carbonates, sulfates, chlorides).
• Capacity: The theoretical maximum amount of material a river can carry, determined by its volume, velocity, and flow regime.
• Competence: Indicates the maximum size of the clast (particle) that a current can transport as bed load.

The Hjulström Curve

The Hjulström Curve is used to analyze the relationship between particle size and the speed of transport. It features two key curves:

• Erosion Curve: Indicates the approximate velocity needed to erode and transport suspended particles of different sizes.
• Sedimentation Curve: Marks the boundary between transport and deposition. It shows the velocity at which particles of a certain size become too heavy to be transported and are thus deposited.

Evolution of River Systems: Equilibrium Profile and Fluvial Terraces

Rivers tend to acquire a longitudinal profile that represents the minimum energy expenditure. This involves a series of processes that aim to achieve more efficient flow conditions.

The equilibrium profile is the longitudinal profile a river acquires when it primarily produces water transport, with minimal net erosion or sedimentation.

The base level is the lowest point to which a river can erode, where it has lost all its energy. This typically corresponds to its confluence with the sea, a lake, or a reservoir.

If the base level falls (e.g., when the continent rises or sea level falls), retrogressive erosion occurs, gradually progressing upstream towards the river's headwaters. Conversely, if the base level rises, retrogressive deposition occurs.

Other effects of retrogressive erosion include fluvial capture processes.