River Deposition — Depositional Landforms

A river is always carrying a load of boulders, pebbles, sand, silt and dissolved material. The lesson on river erosion explains how that load is worn away and moved. Sooner or later, much of it is dropped, and the places where it is dropped build the landforms of the middle and lower course.

A river's energy comes from its moving water. Two things matter most. The first is velocity, how fast the water flows. The second is discharge, how much water is flowing.

The size of the load matters too. A large boulder needs far more energy to move than a grain of sand. When a river slows, the biggest and heaviest particles are dropped first. Fine sand and silt can stay suspended in the water for much longer, so they travel further before they settle.

Putting these together, a river deposits when its energy falls below what its load needs. This happens when:

• velocity drops, for example where the gradient flattens, on the inside of a bend, where water spills out over flat land, or where the river enters the sea
• discharge falls, so there is less water to push the load along
• the load is too large or too heavy for the energy available

Erosion, transportation and deposition all vary with the same set of factors:

• position along the river course
• velocity of the flow
• size of the load
• discharge
• local rock type
• pH of the river water (which affects how much rock dissolves)

Because these factors change along a river, the balance between the three processes changes too. Near the source, steep gradients favour erosion. Further downstream, deposition becomes more and more important.