River Physical Processes

Mass Movement Processes

• A dry slide down a grassy hill might just give you grass stains, but a saturated, muddy slope can pull down entire trees. Water drastically changes how gravity affects a valley side.
• Mass movement is the downhill movement of weathered material under the force of gravity. Any slope with a gradient steeper than 5° is susceptible.
• Mass movement supplies sediment to the river, providing the abrasive tools it needs for future erosion.

There are two main types of mass movement relevant to river landscapes:

• Sliding: The rapid downhill movement of a large block of rock or soil en masse along a flat slip plane (like a bedding plane). The material remains largely intact and does NOT rotate.
• Slumping: The downward movement of saturated material along a curved or concave slip plane.

Step-by-Step Describe: Slumping / Rotational Slip

1. First, heavy or prolonged rainfall infiltrates a permeable rock layer or soil.
2. Then, the water hits an underlying impermeable layer (like clay), causing the upper layer to become completely saturated.
3. Because of this saturation, the material's weight drastically increases, and the water acts as a lubricant, reducing internal friction.
4. Meanwhile, the river erodes and undercuts the base of the valley side, removing support for the slope above.
5. Finally, the heavy, unsupported material collapses and moves rotationally backwards down the concave slip plane, leaving a crescent-shaped scar.

River Erosion Processes

• Understanding how a river spends its energy explains why most of its power is actually wasted. Approximately 95% of a river's energy is used purely to overcome friction against its bed and banks, leaving just 5% available to erode and transport material.
• Vertical erosion acts downwards, deepening the channel (dominant in the upper course).
• Lateral erosion acts sideways, widening the channel (dominant in the middle and lower courses).

Rivers erode their channels and load using four specific mechanisms:

• Hydraulic action: The sheer force of fast-flowing water hits the river banks and bed. It forces air into cracks, compressing the air and creating intense pressure that eventually fractures the rock.
• Abrasion: The sandpapering effect where the river's load scrapes, rubs, and bangs against the bed and banks, wearing them away.
• Attrition: Stones and rocks being carried by the river collide with each other, knocking off jagged edges to become smaller and rounder. Crucially, attrition does NOT erode the river channel itself.
• Corrosion: The chemical process where slightly acidic river water reacts with and dissolves soluble minerals in rocks.

Step-by-Step Describe: Waterfall Formation

1. First, a river flows over a band of resistant (hard) rock overlying less resistant (soft) rock.
2. Then, hydraulic action and abrasion erode the soft rock faster, creating a step in the riverbed.
3. Over time, continued erosion undercuts the hard rock, creating a plunge pool and a protruding overhang.
4. As a result of gravity, the unsupported overhang eventually collapses, providing abrasive rock tools that fall into the plunge pool.
5. Finally, this continuous process causes the waterfall to retreat upstream, leaving a steep-sided gorge behind it.

River Transportation Processes

• Why do some rivers look perfectly clear while others look thick and muddy? It all comes down to the size of the load and how the water moves it.
• A river's transport ability is directly proportional to its velocity (speed) and discharge (volume of water).
• Sediment becomes progressively smaller and rounder downstream due to attrition.

Rivers transport their load in four distinct ways:

• Traction: Large boulders and heavy rocks are rolled or pushed along the riverbed by the force of the water.
• Saltation: Small pebbles and stones are bounced or hopped along the riverbed. The water momentarily lifts them before dropping them back down.
• Suspension: Fine, light particles are carried within the flow of the water, held up by turbulence.
• Solution: Dissolved minerals are carried invisibly in the water.

The material transported by traction and saltation is called the bedload. Material transported via suspension is the suspended load, and dissolved minerals form the dissolved load.

River Deposition Processes

• Every time you ride a bike up a steep hill and then reach a flat path, your speed drops instantly. Rivers behave the exactly same way when they reach flatter land or enter a lake—they lose speed and drop what they are carrying.
• Deposition occurs specifically when a river loses energy due to a decrease in velocity or discharge.
• Rivers deposit material strictly in order of weight and size. The heaviest bedload is dropped first, while finer sediment (collectively called alluvium) is carried the furthest.

Conditions that trigger a loss of energy and subsequent deposition include:

• Decreased gradient (flattening slopes).
• Flowing on the inside bends of meanders, which are furthest from the thalweg (the line of fastest flow).
• Entering standing water like a lake or the sea. Here, flocculation can occur, where saltwater causes fine clay particles to clump together into heavier masses that quickly settle.
• Flooding across a floodplain, which massively increases the wetted perimeter (the area of land contacting the water), causing a sudden spike in friction.

Step-by-Step Describe: Levee Formation

1. First, during a period of high discharge, a river overflows its banks and spreads water across the flat floodplain.
2. Then, the water immediately loses velocity due to massively increased friction with the land.
3. Finally, this sudden loss of energy causes the heaviest load to be deposited first, directly at the river's edge, building up a natural raised embankment called a levee.