River Profiles and Fluvial Processes

Changes from Source to Mouth

• In the upper course, the river has high potential energy due to altitude. The valley is steep-sided and a V-shaped valley, while the channel is narrow (often under $1 - 2\,\text{m}$ wide) and shallow, filled with large, angular bedload.
• As the river enters the middle course, the gradient becomes gentler, and the river begins to erode sideways (lateral erosion), creating gentler valley sides and a narrow floodplain. The channel becomes wider and deeper (over $1\,\text{m}$).
• In the lower course, the gradient is almost flat. The valley becomes very wide and flat-bottomed with indistinct sides, and the channel reaches its widest and deepest point (sometimes over $1\,\text{km}$ wide).

The River Tees Case Study

• The River Tees starts at its source on Cross Fell at an altitude of $893\,\text{m}$ and flows for $137\,\text{km}$ to the sea.
• A famous feature in its upper course is High Force Waterfall. It has a $21\,\text{m}$ drop and represents a major knick point in the river's long profile.

Calculating River Gradient

To understand how steep a river is, you can calculate its gradient using the formula:

$$\text{Gradient} = \frac{\text{Change in height}}{\text{Horizontal distance}}$$

Step 1: Identify the start and end heights and distance.

• Example: A river falls from $400\,\text{m}$ to $100\,\text{m}$ over a horizontal distance of $10\,\text{km}$ ($10{,}000\,\text{m}$).

Step 2: Calculate the vertical interval (change in height).

• Change in height = $400\,\text{m} - 100\,\text{m} = 300\,\text{m}$

Step 3: Substitute the values into the formula.

• $\text{Gradient} = \frac{300\,\text{m}}{10{,}000\,\text{m}}$

Step 4: Calculate the final answer.

• $\text{Gradient} = 0.03$ (This can also be expressed as a ratio of 1 in 33.3).

The Formation of a V-Shaped Valley

The steep, narrow valleys of the upper course form through a sequence of step-by-step processes:

1. Vertical erosion: The river cuts downwards into the bedrock using hydraulic action and abrasion.
2. Weathering: The exposed valley sides are weakened by sub-aerial processes, such as freeze-thaw weathering.
3. Mass movement: The weakened material moves down the slopes via soil creep or slumping.
4. Transport: The river carries the fallen material away downstream, leaving behind the characteristic V-shaped valley.

How Rivers Erode

• Did you know that 95% of a river's energy is used just to overcome friction? Only 5% is left over for erosion and transportation, with the most erosion happening during floods when discharge is highest.
• In the upper course, vertical erosion (downward cutting) is dominant. In the middle and lower courses, lateral erosion (sideways cutting) takes over, especially on the outside bends of meanders.
• Hydraulic action occurs when the sheer force of the water hits the river banks and bed, forcing air into cracks. The trapped air is compressed, causing the rock to shatter and break apart over time.
• Abrasion happens when the river's load of stones and pebbles scrapes along the bed and banks like sandpaper, wearing them away.
• Attrition is when rocks and stones carried by the river collide with each other, causing them to break into smaller, smoother, and rounder pieces. Note that this erodes the river's load, not the river channel itself.
• Solution (as an erosion process) is a chemical action where acidic river water dissolves soluble minerals in the rocks, such as calcium carbonate found in limestone or chalk.

Waterfall Retreat and Gorge Formation

1. A river flows over a layer of hard rock that sits on top of softer rock.
2. The soft rock erodes faster through hydraulic action and abrasion, creating a step in the riverbed.
3. Continued erosion undercuts the hard rock above, creating an overhang and carving out a deep plunge pool at the base.
4. Eventually, the overhang becomes unsupported and collapses; over time, the waterfall retreats upstream, leaving behind a steep-sided gorge.

How Rivers Transport Material

• Think of how much harder it is to carry a heavy bowling ball compared to a tennis ball. The way a river transports its load depends heavily on the size of the particles and the energy available.
• Larger particles require more energy to move, so a river's transportation capacity increases significantly when its water velocity is high.
• Traction involves large boulders and heavy rocks being rolled along the riverbed. This requires the most energy and occurs mainly in the upper course or during floods.
• Saltation happens when smaller pebbles and stones are bounced along the riverbed in a hopping motion.
• Suspension occurs when fine, light material (like silt and clay) is carried within the water flow. This is the primary mode of transport in the lower course and often makes the water look cloudy.
• Solution (as a transportation process) happens when dissolved minerals are carried invisibly in a chemical state within the water.

Why Rivers Deposit Material

• Dropping heavy shopping bags when you run out of energy is exactly how a river behaves with its sediment. Deposition occurs whenever the river experiences a reduction in velocity, leading to a loss of energy.
• When energy is lost, the river sorts the material it drops by size. The heaviest and largest rocks (like boulders) are deposited first, while finer sediment is carried the furthest before settling.
• A river can lose energy for several reasons, such as when the gradient becomes reduced or when it enters standing water, like a lake or the sea.
• Energy is also lost through increased friction, which happens in shallow water (like the inner bends of meanders), during low discharge, or when floodwater spreads out, increasing the wetted perimeter.
• In estuaries, incoming tidal salt water meets outgoing fresh river water, slowing the flow. A chemical process called flocculation also occurs here, where the salt causes clay particles to clump together, become heavier, and sink.

Features of Deposition

• A slip-off slope is a gentle, beach-like feature that forms on the inner bend of a meander, where the water is shallowest, friction is highest, and velocity is slowest.
• When rivers flood in the lower course, they deposit alluvium, which is fine-grained, highly fertile soil made of silt and clay. This builds up the floodplain over time.

The Formation of Levees

Levees are naturally raised embankments found along the banks of rivers in the lower course. They form through repeated deposition during flood events:

1. When a river experiences high discharge, it overflows its banks onto the floodplain.
2. As the water spills out of the channel, it encounters increased friction from the land, causing a sudden reduction in velocity.
3. Because of this loss of energy, the river deposits its heaviest and coarsest material first, right next to the river banks.
4. Finer sediment is carried further across the floodplain; over many floods, the coarse material builds up to form raised levees.