River processes

• Erosion is the process that wears away the river bed and banks.
• Erosion also breaks up the rocks that are carried by the river.
• There are four types of erosion: Hydraulic action, Abrasion, Attrition, and Solution.
• Hydraulic action is the sheer power of the water as it smashes against the river banks.
• Air becomes trapped in the cracks in the rock of the river bank and bed, and causes the rock to break apart.
• Abrasion is when pebbles grind along the river bank and bed in a sand-papering effect.
• Attrition is when rocks that the river is carrying knock against each other, breaking apart to become smaller and more rounded.
• Solution is when the water dissolves certain types of rocks, such as limestone.
• Weathering and mass movement occur as the river flows, eroding the land and creating a valley.
• A low area of land between hills or mountains, typically with a river flowing through it, is called a close valley.
• A v-shaped valley is a valley with steep sides.
• Soil creep is a very slow movement, occurring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods.
• When the soil dries out, it contracts vertically.
• When wet, soil particles increase in size and weight, and expand at right angles.
• Landslides are occasional, rapid movements of a mass of earth or rock sliding along a steep slope, which can occur after periods of heavy rain, when the water saturates overlying rock, making it heavy and liable to slide.
• There are three types of weathering: Mechanical, which involves changes caused by temperature, such as onion skin weathering; Chemical, which involves rainwater being slightly acidic and wearing away the rock; and Biological, which involves rocks being worn away by plants and animals, for example, roots growing in cracks in the rock or animals such as rabbits burrowing into rock.
• Mass movements can be rapid, such as landslides, or slow as with soil creep.
• As a result, the soil slowly moves down slope.
• The broken rock fragments from weathering move down the slope through mass movements.
• If the river meets more resistant rock it will flow around the rock, producing interlocking spurs.
• Hill that a river meanders around in a V-shaped valley is called an interlocking spur.
• When viewed from downstream, these spurs appear to be locked together.
• The river erodes the valley bottom, but the valley sides are broken down by weathering.