Deposition (Why Rivers Deposit Sediment)

Causes of Deposition

Deposition happens when a river loses the energy needed to carry sediment, causing it to drop the material it’s transporting. Several factors cause this loss of energy:

• Decrease in river velocity: When the river slows down, it can no longer carry as much sediment. This often happens when the gradient (slope) of the river decreases, such as when the river reaches flatter land. Larger, heavier sediment is deposited first as velocity decreases, followed by finer particles.
• Reduction in discharge: Discharge is the volume of water flowing in the river. When discharge falls, there is less water to carry sediment, so deposition occurs. This can happen during dry periods or when tributaries stop adding water.
• Increase in friction: Friction between the water and the riverbed or banks slows the river down. This is common where the river channel becomes rougher or narrower, causing sediment to settle.
• Obstacles in the river channel: Features like large boulders, fallen trees, or human structures can block the flow, reducing velocity and causing sediment to be deposited around these obstacles.

For instance, when a river flows from steep upland areas to the gentler lowlands, the velocity decreases sharply, leading to deposition of larger sediment like gravel and pebbles.

Where Deposition Occurs

Deposition happens in specific parts of a river where conditions encourage sediment to settle:

• Inside bends of meanders: Water moves slower on the inside of bends due to less centrifugal force, so sediment is deposited here, forming slip-off slopes.
• River mouth and estuaries: When a river meets the sea or a lake, the flow slows dramatically, causing large amounts of sediment to be deposited, often forming deltas or mudflats.
• Areas where the river widens: When the channel widens, the water spreads out and slows down, leading to deposition.
• Lower course of the river: Near the river’s end, the gradient is very gentle and velocity is low, so fine sediments like silt and clay settle out, creating wide floodplains.

For example, the River Thames deposits sediment in its estuary, creating mudflats and salt marshes important for wildlife.

Effects of Deposition

Deposition shapes the river landscape and affects the environment in several ways:

• Formation of landforms: Deposition creates features like levees—raised banks formed by sediment deposited during floods—and floodplains, which are flat areas of land beside the river made from layers of deposited sediment.
• Build-up of sediment layers: Over time, sediment accumulates, raising the riverbed and changing the channel’s depth and shape. This can affect how the river floods and flows.
• Changes to river channel shape: Deposition can cause the river to become shallower or split into multiple channels (braiding). It can also lead to the formation of features like oxbow lakes (formed when deposition cuts off meanders, see other notes).

For example, after heavy rainfall, rivers often flood their banks, depositing sediment that builds up the floodplain and levees, making the area fertile for farming.