Most of a river's energy is used to overcome friction
Velocity
The speed of flow
Velocity increases slightly downstream but varies considerably within the river channel itself
Velocity can vary day-to-day and between different seasons, e.g. a torrential downpour or slow melt in late spring/early summer can lead to a sudden increase in river velocity
River erosion
The river erodes the bank and bed of its channel through a number of processes
River erosion processes
1. Hydraulic action
2. Abrasion/corrasion
3. Attrition
4. Solution
Hydraulic action
The first process of the river water itself acting on the bed and bank. It is powerful when a river flows very fast and becomes turbulent. The water looks white because it is full of air bubbles which produce sudden pressure changes when imploded, which can break up the rock.
Abrasion/corrasion
The process by which stones, sand grains and other particles carried by the river are thrown against the bed and banks and help to erode them.
Attrition
Stones and rocks carried downstream are thrown against each other, gradually becoming more rounded as they are carried downstream.
Solution
River water can dissolve some of the minerals in rocks, soil and weathered material. Solution is particularly important in limestone areas, but all rivers carry significant quantities of dissolved salts.
