River Processes and Landforms

How a river’s processes work together

A river shapes its landscape by eroding, transporting and depositing material. These are connected processes, not separate jobs confined to different sections of the river. Material eroded from a bank becomes part of the river’s load: the sediment and dissolved substances it carries. Some of that load helps erode the channel further, and some is eventually deposited.

The balance between these processes depends on the flow and the material available. Faster, more turbulent water can usually move larger particles. If the flow slows sufficiently, particles that it could previously carry settle out. Erosion and deposition can therefore happen close together, even on opposite sides of the same bend.

Four ways rivers erode

Hydraulic action is erosion by the force of moving water. Water dislodges loose material from the bed and banks, especially during powerful flows.

Abrasion happens when the river’s load scrapes, scours or strikes the bed and banks. Imagine stones being dragged across a rocky river bed: the stones act as tools that wear the channel away.

Attrition changes the load itself. Transported fragments collide with one another and the bed, becoming smaller, smoother and more rounded. Unlike abrasion, its main result is the wearing down of the particles rather than the channel.

Solution occurs when soluble minerals dissolve in river water. Weakly acidic water can dissolve carbonate rocks such as limestone and chalk. The dissolved material is then carried away.

Erosion can cut downwards, deepening the channel, or sideways, widening it and helping bends migrate across a valley floor.

Carrying and depositing the load

A river transports material in four main ways:

• Traction: large stones and boulders roll along the bed, usually during high-energy flows.
• Saltation: smaller stones bounce along the bed.
• Suspension: fine particles, such as silt, are carried within the water. Suspended sediment can make a river look brown.
• Solution: dissolved minerals travel with the water. They are not visible particles floating in it.

Transported stones can abrade the bed and banks while also undergoing attrition. Transport therefore helps erosion continue.

Deposition occurs when the river can no longer carry part of its load. This can happen in slower water on the inside of a bend, when floodwater spreads beyond the channel, or where a river enters a lake or sea. Larger, heavier particles generally settle first, while finer material travels further. Not all material is deposited at once, and dissolved minerals do not settle like stones or sand.

Waterfalls: erosion, collapse and retreat

Waterfalls commonly develop where resistant rock lies above less resistant rock. The less resistant rock erodes more quickly, creating a step in the river bed. Water falls over the resistant rock into the channel below.

Hydraulic action and abrasion deepen a plunge pool at the foot of the fall. They also erode the less resistant rock beneath the resistant layer, undercutting it and leaving an overhang. Eventually, the unsupported rock collapses. Fallen blocks can be moved around in the plunge pool, helping to abrade it further.

Repeated undercutting and collapse make the waterfall retreat upstream. A narrow, steep-sided gorge is left along its former position. Transport matters here because moving rock fragments provide the tools for abrasion.

Interlocking spurs: downcutting around resistant ridges

Interlocking spurs are projecting ridges of valley-side land that alternate from opposite sides of a narrow valley. Viewed from above, they appear to fit between one another rather than forming a straight, open valley floor.

In the upper course, erosion is predominantly vertical: hydraulic action deepens the channel, while lateral erosion is limited. The river follows a winding route around more resistant rock rather than cutting sideways through it. As the river cuts down, these projections remain on alternating sides of the narrow, steep-sided valley, forming interlocking spurs. Weathered material falling from the valley sides can be carried away by the river.

Further downstream, lateral erosion and migrating meanders can wear away projecting valley sides. This helps widen the valley and creates space for a floodplain. The important contrast is between a narrow valley with projecting spurs and a broad valley floor shaped by sideways erosion and deposition.

Meanders: erosion outside, deposition inside

A meander is a bend in a river. Flow is usually faster on the outside of the bend, where the channel is deeper. Hydraulic action and abrasion undercut the outer bank, producing a steep river cliff.

On the inside of the bend, water is slower and shallower. Sediment is deposited, building a gently sloping slip-off slope, also called a point bar. The channel is therefore asymmetrical: deeper on the outside and shallower on the inside.

Water also moves through a bend in a corkscrew-like pattern called helicoidal flow, helping transfer sediment towards the inner bank. Together, outer-bank erosion and inner-bank deposition make the bend change position over time.

Oxbow lakes: cutting off a bend

As erosion continues on the outer banks of a pronounced meander, the narrow strip of land between neighbouring bends—the neck—becomes narrower.

During a high flow, often a flood, the river can cut through this neck. Water follows the new, shorter channel rather than travelling around the whole bend. Deposition then blocks the entrances to the abandoned loop, isolating it as an oxbow lake.

Both erosion and deposition are necessary: erosion opens the shortcut, while deposition seals off the old bend. An oxbow lake may later fill with sediment and dry out, leaving a curved meander scar.

Floodplains and natural levees

A floodplain is the relatively flat valley floor beside a river. Its formation combines lateral erosion with deposition. Migrating meanders widen the valley floor, while floods spread water and sediment across it. As floodwater slows, it deposits alluvium, gradually building up the floodplain.

When water first spills over the banks, the flow slows and the coarser part of the overflow’s sediment load is deposited near the channel. Finer silt and clay can travel further across the floodplain before settling.

Repeated floods build raised banks of sediment beside the channel. These are natural levees. A levee is therefore a depositional feature produced by repeated flooding, whereas the floodplain as a whole reflects both erosion and deposition.

Deltas: deposition at a river mouth

A delta develops where a river enters a sea or lake and deposits sediment. As the flow spreads and slows, its ability to carry sediment decreases. Larger particles are generally deposited nearer the mouth, while finer material travels further.

In seawater, fine clay particles can clump together through flocculation, helping them settle. As sediment accumulates, the river may divide into distributaries: channels that branch away from the main river.

A delta forms only if deposition is faster than sediment removal. A large sediment supply and weak removal by waves, tides and currents favour its growth. A river mouth does not automatically have a delta.

Recognising landforms on OS maps

Start with the river’s shape, then check the contours and labels around it. Contours join places of equal height. Closely spaced contours indicate steep slopes; widely spaced contours indicate gentler slopes. Always read the map’s key and contour interval before interpreting the detail.

Meanders appear as winding bends in the river channel. A separate curved water body beside a bend may be an oxbow lake, particularly if its shape resembles an abandoned loop. Its position and shape are evidence; not every nearby pond is an oxbow lake.

A floodplain is suggested by a broad, low, relatively flat area beside the river, with widely spaced contours. Closely packed contours along the valley sides can help show where that flat floor ends. Do not infer a floodplain from an empty area of map alone.

For interlocking spurs, trace alternating projections of higher ground from opposite valley sides, with the river winding between them. Unlike a meander on a broad floodplain, these bends fit around valley-side ridges.

A waterfall may be identified by a name or label, supported by evidence of a steep valley. For example, High Force is a waterfall name. A narrow, steep-sided valley downstream can support identification of a gorge, but steep slopes alone do not prove that a gorge was formed by a retreating waterfall.

In the example, the connected looping channel indicates meanders. The detached curved water body suggests an oxbow lake. Widely spaced contours around these features indicate gentle ground consistent with a floodplain; tightly packed contours farther from the channel indicate steeper valley sides.

Transfer this approach to a real OS map by checking its scale, key, contour heights and labels. The sketch demonstrates how clues combine; it is not a real location or a reproduction of official OS symbols. Small features such as slip-off slopes and natural levees may not be distinguishable at the map’s scale, so identify only what the mapped evidence supports.

Processes

• Erosion: hydraulic action dislodges material; abrasion wears the bed and banks; attrition rounds and reduces the load; solution dissolves soluble minerals.