How Do Streams & Rivers Cause Erosion & Deposition?

Detachment

• Detachment is the process of breaking the bond between soil particles before it can be lifted and carried away. Abrasion is typically the detaching force in streams and rivers, as water flows against the particle. The force must be strong enough to overcome particle cohesion, which varies according to the type of sediments and the kind of bonds between the particles. The bonds between particles in igneous rocks, for example, are much stronger than those in sedimentary rocks.

Entrainment

• Entrainment is the act of a particle being lifted away from its substrate. In the case of rivers and streams, this is often hard to distinguish from detachment, as the flowing water tends to instantly entrain a particle once it has been detached. Fluid drag is the name of the entrainment agent at work in streams, as the water pulls the loose particles away from a bank or channel side. Fluid drag is the strongest at the inside part of a curving channel, since water velocity is highest there.

Transport

• Once the particle has been detached and entrained, a river will transport it to another location. There are four modes of transport that can occur within a stream bed. The particle can be suspended, in which case it is carried along in the water without ever reaching the surface or touching the bottom, or it can be moved with saltation, which means the sediment cycles from the surface to the bottom in "bouncing" motions. Larger particles will roll or slide across the bottom in a process called traction, while extremely small particles can dissolve into the water itself.

Deposition

• Deposition from streams and rivers occurs when the water velocities decrease to the point where it can no longer transport the sediment. The outside curves of stream beds often have sandbars, because the water velocity has decreased enough at those points to drop its particles. Deposition also occurs when streams enter a delta or the ocean. The largest particles are deposited first, due to the higher resistance they cause to transportation. Local deposition can also occur when plants, rocks or other objects slow the water.