Abrasion. Abrasion is the process by which a stream’s irregular bed is smoothed by the constant friction and scouring impact of rock fragments, gravel, and sediment carried in the water. The individual particles of sediment also collide as they are transported, breaking them down into smaller particles. Generally the more sediment that a stream carries, the greater the amount of erosion of the stream’s bed. The heavier, coarser‐grained sediment strikes the stream bed more frequently and with more force than the smaller particles, resulting in an increased rate of erosion.
Circular depressions eroded into the bedrock of a stream by abrasive sediments are called potholes. The scouring action is greatest during flood conditions. Potholes are found where the rock is softer or in locations where the flow is channeled more narrowly, such as between or around boulders.
Solution. Rocks susceptible to the chemical weathering process of solution can be dissolved by the slightly acidic water of a stream. Limestones and sedimentary rock cemented with calcite are vulnerable to solution. The dissolution of the calcite cement frees the sedimentary particles, which can then be picked up by the stream’s flow through hydraulic action.