1. Gravity: Heavier sediments have a higher gravitational force acting on them. As a river's current slows down, its ability to carry heavier particles decreases. The heavier particles settle first, closer to the source of the river where the current is stronger.
2. Settling Velocity: Larger and denser particles (heavier sediments) have a faster settling velocity compared to smaller and lighter particles. This means they sink to the riverbed more quickly as the current weakens.
3. Turbulence: Turbulent water can keep even heavy particles suspended. However, as the river enters calmer waters, the turbulence decreases, allowing heavier particles to settle first.
4. Size and Shape: The shape and size of the particles also play a role. Larger and more irregularly shaped particles have a greater surface area for resistance and are more likely to settle first.
5. River Gradient: The slope of the river bed (gradient) influences the flow velocity and thus the settling of sediments. A steeper gradient leads to faster currents and carries heavier particles further downstream.
Example:
Imagine a river flowing from mountains to a flat plain. As the river enters the plain, its flow slows down. The heavier sediments like gravel and sand settle first near the mountains, while the lighter sediments like silt and clay are transported further downstream.
Therefore, the combination of gravity, settling velocity, turbulence, size, and gradient leads to the deposition of heavier sediments first in rivers. This phenomenon results in the characteristic layering of sediments observed in riverbeds, with coarser sediments closer to the source and finer sediments further downstream.
