When the gradient of a river channel changes from a steep to a gentle angle, the main activity of the river will be

When a river's gradient changes from steep to gentle, its velocity decreases. A river's ability to transport sediment depends on its velocity and energy. As the gradient flattens and the water slows down, the river loses energy and can no longer carry its full sediment load. The river therefore begins depositing material.

This is a direct application of the Hjulstrom curve principle: faster-flowing water erodes and transports, while slower-flowing water deposits. The transition from a steep to a gentle gradient is exactly the condition that triggers deposition.

Common real-world examples of this process include:

• Alluvial fans - formed where a mountain stream meets a flat plain
• Flood plains and deltas - formed where rivers slow down on reaching low-lying areas or the sea
• Levees - built up when a river overflows its banks and immediately loses velocity

Erosion and deepening are dominant processes on steep gradients where the river has high velocity and energy. Terracing is an agricultural land-management technique, not a natural river process.