Streams are one of the most effective surface agents that erode rock and sediment. Erosional landscapes such as the Grand Canyon have been formed by constant erosion from running water over millions of years. In addition to eroding the bedrock and previously deposited sediments along its route, a stream constantly abrades and weathers the individual rock and soil particles carried by its water. Hydraulic action, abrasion, and solution are the three main ways that streams erode the earth's surface.
Hydraulic action. The ability of flowing water to dislodge and transport rock particles or sediment is called hydraulic action. In general, the greater the velocity of the water and the steeper the grade, the greater the hydraulic action capabilities of the stream. Hydraulic action is also enhanced by a rough and irregular stream bottom, which offers edges that can be “grabbed” by the current and that create uplifting eddies.
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.