Rivers erode in four ways:
Abrasion or corrasion - This is when large pieces of bedload material wear away the river banks and bed.
Attrition - This is when the bed load itself is eroded when sediment particles knock against the bed or each other and break, becoming more rounded and smaller.
Hydraulic Action - This is when the force of water erodes softer rock.
Solution or corrosion - This is when acidic water erodes rock.