In a straight river channel the waters highest velocity is in the middle and just below the surface. If anything disrupts this e.g. large boulder or softer sediment, it will divert around the obstacle creating changes in speed and depth of the river (1).

This change in direction forms pools (deeper water and greater erosion) (1) and riffles (areas of shallow water with less energy and greater turbulence) (1). Once these form rivers flow from side to side in a winding course.

The outside of a bend has faster moving water so has more energy to erode(1). Hydraulic action(1), when air is compressed into the river bank causing materials to be dislodged(1) and Abrasion(1), pieces of broken-off rock thrown at the river bank causing erosion(1), occur here creating a river cliff. (1)

As the material is eroded at one bank a corkscrew-like motion of water known as helicoidal flow moves the eroded material from the outside of one meander and deposits it on the next. (1)

Water moves slower on the inside bend of a river, losing energy it deposits its load forming a river beach. (1)

Continuous erosion and deposition will create wider and wider meanders, which migrate downstream and outwards across floodplains in lateral erosion. (1)