Landforms and stages of a river
A landform is a natural shape on the Earth's surface, like a valley, hill or dune. Each agent (running water, wind, glaciers, waves, groundwater) makes its own set of landforms by erosion (wearing away) and deposition (dropping material). Landforms change slowly through stages: youth, maturity and old age.
| Stage | River | Landforms |
|---|---|---|
| Youth | Steep slope, fast flow, few streams | V-shaped valleys, gorges, waterfalls, rapids |
| Maturity | Many streams, wider valleys | Wide V-valleys, flood plains begin, meanders |
| Old age | Gentle slope, slow flow | Wide flat flood plains, large meanders, oxbow lakes, levees, deltas; peneplain (almost-plain) with a few leftover hills (monadnocks) |
In humid areas, running water is the most important agent. Two ways it erodes: overland flow as a sheet (sheet erosion) and linear flow in channels, which starts as small rills and grows into gullies and valleys.
Erosional landforms of running water
| Landform | How it forms |
|---|---|
| Valleys | Rills widen into gullies, then into valleys. Deep downward cutting makes a V-shaped valley. |
| Gorge | Very deep valley with very steep, almost vertical sides; about as wide at the top as at the bottom. Forms in hard rock. |
| Canyon | Steep, step-like side slopes; wider at the top than at the bottom. Forms in horizontal layers of sedimentary rock. |
| Potholes | Round holes in the rocky bed, drilled by pebbles swirling in eddies. |
| Plunge pools | Large, deep holes at the foot of waterfalls, dug by falling water and boulders. |
| Incised or entrenched meanders | Deep, winding gorges cut into hard rock when a meandering river's land is uplifted and it cuts down. |
| River terraces | Flat steps along valley sides: old valley floors or flood plains left high after the river cut deeper. Paired at the same height on both sides, or unpaired. |
Waterfall: where hard rock lies over soft rock, the soft rock wears faster and the river falls over the edge.
Depositional landforms of running water
When a river slows down (gentle slope, entering a plain or the sea), it cannot carry its load and drops it: coarse material first, fine later.
| Landform | How it forms |
|---|---|
| Alluvial fan | A fast stream from hills reaches a flat plain, slows suddenly and spreads a fan-shaped, cone-like heap of coarse load. |
| Delta | At the mouth, the river meets calm sea or lake water and drops fine sediment in layers; the river splits into many channels (distributaries). |
| Flood plain | Flat land beside the river built by sediment dropped during floods. Active (floods now) or inactive (older, higher). |
| Natural levees | Low ridges of coarse sediment along the banks, dropped right beside the channel during floods. |
| Point bars | Sand and gravel deposits on the inner (convex) side of a meander bend. Also called meander bars. |
| Braided channels | When load is too much, the river splits around sand bars and islands into many threads. |
Meanders and oxbow lakes
On gentle slopes a river begins to loop. A meander is a big loop-like bend (named after the winding Menderes river in Türkiye).
- Water flows faster on the outside of a bend, so it cuts the outer (concave) bank and makes a steep cut bank (undercut bank).
- On the inside, water is slow, so it drops sand as a point bar on the gentle (convex) bank.
- The loop becomes more curved and its neck narrows.
- During a flood the river cuts straight across the neck.
- The old loop is left as a curved, still lake: an oxbow lake (shaped like the U-shaped collar of an ox yoke).
Try it at home: pour water slowly along a sand tray with a gentle slope. Watch small bends appear, the outer edges wash away and sand pile on the inner edges.
Landforms made by wind: erosion
In hot deserts, few plants hold the soil and big temperature changes break rock, so wind and occasional sheet floods after rare storms do most of the work.
Wind erodes in two main ways: deflation (lifting and blowing away loose dust and sand) and abrasion (sand grains in the wind blasting rock like sandpaper, strongest near the ground).
| Landform | What it is |
|---|---|
| Pediments | Gently sloping rocky floors at the foot of mountains, formed as the mountain face wears back. |
| Pediplains | Nearly flat plains formed when pediments join as mountains wear down. |
| Inselbergs | Leftover isolated hills standing on the pediplain (island-mountains). |
| Playas | Flat basins where rain water collects and evaporates, leaving salt (alkali flats). |
| Deflation hollows | Shallow pits scooped out by wind blowing away loose sand. |
| Blow-outs and caves | Hollows deepened by wind become caves in rock faces. |
| Mushroom, table and pedestal rocks | Rocks with a narrow base and wide top, because sand-blasting is strongest near the ground. |
Landforms made by wind: deposition (sand dunes)
Wind sorts sand well. When it slows down (behind a bush or rock) it drops sand in heaps called sand dunes. Their shape depends on wind direction, wind strength, sand supply and plants.
| Dune | Shape and conditions |
|---|---|
| Barchan | Crescent; horns (wings) point away from the wind (downwind); steady wind, moderate sand. |
| Parabolic | Crescent reversed: horns point towards the wind; forms when plants partly cover the sand. |
| Seif | Like a barchan with only one long wing; a long, sharp ridge, when wind direction changes. |
| Longitudinal | Long ridges parallel to the wind; low sand supply, steady wind. |
| Transverse | Ridges at right angles to the wind; lots of sand, steady wind. |
Fine dust carried far by wind and dropped in thick layers is called loess.
Key formulas and definitions
- Stages: youth (cutting) → maturity (widening, meanders) → old age (deposition, flood plains, oxbow lakes)
- River erosion: V-valley, gorge (steep, same width top/bottom), canyon (step-like, wider top), pothole, plunge pool, incised meander, terrace
- River deposition: alluvial fan, delta, flood plain, natural levee, point bar, braided channel
- Oxbow lake: outer bank cut + inner bank point bar → neck cut → loop left as lake
- Wind erosion: deflation + abrasion → pediment, pediplain, inselberg, playa, deflation hollow, mushroom rock
- Dunes: barchan (horns downwind), parabolic (horns upwind), seif (one long wing), longitudinal (parallel), transverse (across)
Worked examples
1. A deep valley has nearly vertical walls and is as wide at the top as at the bottom. Gorge or canyon?
Gorge. A canyon has step-like sides and is wider at the top.
2. Why do natural levees form right next to the river channel?
When a river floods, water slows immediately as it spills over the bank, so it drops its heaviest, coarse sediment right beside the channel, building a low ridge.
3. Explain the formation of an oxbow lake in four steps.
1) Outer bank is cut, inner bank gets point bars; 2) the loop grows and its neck narrows; 3) a flood cuts through the neck; 4) the old loop is left as a curved lake.
4. A crescent dune has its horns pointing downwind in a region with steady wind. Name it.
Barchan dune.
5. Why are mushroom rocks narrow at the bottom?
Wind carries most sand close to the ground, so abrasion wears the lower part faster than the top.
6. Why does a fast mountain stream make an alluvial fan at the foot of the hills?
When it reaches the flat plain its speed drops suddenly, so it cannot carry its coarse load and spreads it out in a fan shape.
Common mistakes
- Mixing up gorge and canyon. Gorge: steep walls, similar width top and bottom. Canyon: step-like sides, wider at the top.
- Saying the river cuts the inner bank. It cuts the outer bank and deposits on the inner bank.
- Mixing up barchan and parabolic dunes. Barchan horns point downwind; parabolic horns point upwind.
- Thinking wind alone shapes deserts. Rare but powerful sheet floods also do a lot of erosion in deserts.