THE EQUILIBRIUM POINT

THE EQUILIBRIUM POINT

Considering the classic meaning of "EQUILIBRIUM POINT" along a logitudinal profile of a generic river, it is the point (when present), expressed in terms of elevation a.s.l., downstream of which erosive processes cease and the processes of transport and sedimentation by surface runoff begin or are maintained.

This point could be also represented within a summary graphical solution along the HYPSOMETRIC CURVE and is expressed there in terms of hypsometric coordinates: (ae​;he​).

THE TRANSITION POINT

The transition point along a river from erosional to depositional processes has no specific name, but it is a key concept in fluvial geomorphology and is best described by the achievement of the equilibrium profile.

THE EQUILIBRIUM PROFILE

A river tends to reach a theoretical condition called the “equilibrium profile”.

This represents a longitudinal profile (the curve describing the river's elevation from source to mouth) in which the current's energy is just sufficient to transport the sediment load it receives from its drainage basin, without eroding or depositing material along its course.

In this ideal state there is a balance between the drag force of the current and the resistance of the riverbed material.

Net erosion and net deposition are zero.

THE TRANSITION ZONE

In reality, rather than a single "point," there is a transition zone where this change occurs. This zone is typically found where there is a sharp decrease in the slope of the river.

The main features of a river and the dominant processes are:

1. Upper Course (Erosion Zone): Characterized by steep gradients, high energy, and V-shaped valleys, the river has the power to erode the rocks and carve out its own riverbed.

2. Middle Course (Transport and Lateral Erosion Zone): The slope decreases, velocity drops, and the river begins to meander. Erosion is concentrated on the outer banks of the meanders, while selective deposition begins on the inner banks.

3. Lower Course (Deposition Zone): In the plains, the slope is minimal. The river's energy is drastically reduced, no longer able to transport its entire solid load. Sediments, from the coarsest to the finest, are deposited, creating alluvial plains, deltas, and other forms of deposition.

In summary, the transition from erosion to deposition is a gradual process manifested by changes in the river's slope and energy, culminating in the achievement of an equilibrium profile in the lower reaches, where deposition becomes the dominant geomorphological process.