River Landforms

How rivers shape their landscapes

A river changes its landscape by erosion, which removes material, and deposition, which lays material down. The river's load is the sediment it transports. Water can erode directly through hydraulic action, while sediment scraping against the bed and banks causes abrasion. When water slows and can no longer carry part of its load, that material is deposited.

The direction of erosion matters. Vertical erosion cuts down into the riverbed and deepens the valley. Lateral erosion wears away the banks and widens the channel or valley. Vertical erosion is especially important in the upper course, whereas lateral erosion becomes more important in lowland reaches.

Geology also affects the landscape. Some rocks resist erosion more strongly than others. Rivers therefore do not wear away every part of their bed and banks at the same rate. Differences in rock resistance help explain both the routes rivers follow and features such as waterfalls.

Interlocking spurs: a river winding between ridges

In an upland valley, a river cuts downwards through hydraulic action and abrasion. Weathering breaks up rock on the valley sides, and gravity moves loosened material downslope. Together, these processes help produce a steep, V-shaped valley.

The river winds around projecting areas of more resistant high ground rather than cutting straight through them. Because erosion is mainly downwards, with relatively little sideways erosion, the ridges remain prominent. These ridges are interlocking spurs.

Imagine looking upstream along the valley: a ridge projects from the left, then another from the right, and so on. They appear to overlap or interlock, with the river winding between them. The spurs are remaining areas of high land, not piles of sediment deposited by the river.

Waterfalls and gorges: unequal erosion and upstream retreat

A common setting for a waterfall is a layer of more resistant rock overlying less resistant rock. Hydraulic action and abrasion erode the softer rock faster, producing a step in the riverbed over which water falls.

At the base of the fall, turbulent water and swirling rock fragments erode a plunge pool. The softer rock beneath the resistant layer is also worn away, undercutting it and leaving an unsupported overhang.

Eventually, the overhang collapses. The fallen rock fragments provide more material for abrasion in the plunge pool. With the edge of the resistant layer now farther upstream, the waterfall has retreated.

Repeated undercutting and collapse move the waterfall upstream, leaving a narrow, steep-sided gorge downstream. The waterfall and gorge are therefore connected stages of the same process: the gorge marks the route along which the waterfall has retreated.

Meanders: erosion on one side, deposition on the other

A meander is a bend in a river. In lowland areas, lateral erosion and deposition work together to enlarge bends and move the channel across its valley.

On the outside of a bend, the fastest flow erodes the bank through hydraulic action and abrasion. Undercutting leaves the bank unsupported, so it collapses. This produces a steep river cliff, and the outside edge of the bend moves outwards. The resistance of the bank material affects how readily it can be eroded.

On the inside of the bend, slower water deposits sediment. The deposit is a point bar, beside a gently sloping bank known as a slip-off slope.

Picture a cross-section straight across a bend: the outer side has a steep, eroding bank, while the inner side has a gently sloping accumulation of sediment. These are not separate, unrelated features. Erosion removes material from one side while deposition builds up the other, allowing the meander to migrate across the valley.

Oxbow lakes: cutting off a meander

As neighbouring outside bends are eroded, the strip of land between them becomes narrower. This strip is the meander neck.

During a flood, powerful flow can cut through the narrowed neck. Water then follows the shorter, straighter channel rather than travelling around the whole loop.

Flow into the old loop becomes slower, encouraging deposition at its entrance and exit. These deposits eventually seal the loop off from the main channel, leaving a curved oxbow lake.

The order matters: erosion narrows the neck, floodwater cuts through it, and deposition then isolates the former meander. Both erosion and deposition are needed to explain the completed landform.

Floodplains and levees: repeated floods build the landscape

A floodplain is the broad, relatively flat land beside a river. Meanders migrating sideways help widen the valley floor. Deposition during floods then builds up its surface.

When river discharge—the volume of water flowing past a point each second—is high enough, water can overflow the banks. As it spreads over the floodplain, contact with the land surface increases friction and slows the flow. The water loses the ability to carry some of its sediment, so it deposits material across the floodplain.

Repeated floods add further layers of sediment, gradually raising the floodplain surface. Its development therefore combines the sideways movement of the river with deposition outside the channel.

Deposition also produces natural levees. The heaviest material settles first, close to the channel, while finer material can be carried farther across the floodplain. Repeated floods build up raised banks beside the river.

Imagine a cross-section from one side of the valley to the other: a broad floodplain leads up to a raised levee, drops into the river channel, then rises to another levee before extending across the opposite floodplain. Both the levees and the wider floodplain contain deposited sediment, but the levees are concentrated beside the channel.