Erosion, Sediment Transport, and Mass Movement
Erosion is the removal of weathered rock and soil by agents such as water, wind, ice and gravity; sediment transport is the movement of that material, and mass movement is the downslope movement of rock and soil under gravity, such as landslides, rockfalls and soil creep. In brief: rivers move sediment by traction, saltation, suspension and solution, and deposit it when their energy falls. Mass movement can be a slow creep of millimeters per year or a sudden landslide, and is often triggered by heavy rain, earthquakes or undercutting of a slope.
Together these processes wear down mountains, build floodplains and deltas and create hazards. This guide explains the agents of erosion, how sediment is carried, the main types of mass movement, compares them in a table, works through example calculations, outlines the history of the field and corrects common mistakes.
What causes erosion?
Erosion needs energy and a moving agent. Running water (rain splash, sheet wash, streams and rivers) is the most important agent overall. Waves erode coasts, glaciers scrape and pluck bedrock, wind picks up fine sand and dust in dry regions, and gravity pulls material downhill. Rivers erode by hydraulic action (the force of water), abrasion (rock fragments scraping the bed and banks), attrition (fragments knocking against each other and becoming smaller and rounder) and solution (dissolving of minerals such as limestone).
Erosion is faster on steep slopes, where there is little vegetation, where rainfall is intense and where the rock or soil is loose. Plant roots hold soil, and cutting forests or plowing along slopes increases erosion.
How is sediment transported?
A river carries material in four ways, depending on particle size and the speed of the flow.
- Traction: large boulders and pebbles are rolled or dragged along the bed, mainly during floods.
- Saltation: sand-sized grains bounce along the bed in short hops.
- Suspension: fine silt and clay are carried within the water, which makes a river look muddy. This is usually the largest part of the load.
- Solution: dissolved minerals are carried invisibly in the water.
Wind uses the same ideas: dust travels in suspension (sometimes across oceans), sand moves by saltation and larger grains creep along the surface. Glaciers carry material on, within and beneath the ice. Longshore drift moves sediment along coasts. The faster the flow, the larger the particles that can be moved, and the carrying power rises very steeply with velocity.
When is sediment deposited?
Deposition happens when the transporting energy drops: a river slows when it enters a lake or the sea, when it spreads over a floodplain, when its gradient decreases or when the volume of water falls. The heaviest particles are dropped first and the finest last, so sediment becomes sorted by size. The result is landforms such as floodplains, levees, deltas, alluvial fans and sandbars. Wind drops sand when it meets an obstacle, building dunes, and glaciers drop unsorted material, called till, when they melt.
What is mass movement?
Mass movement is the downslope movement of rock, regolith and soil caused mainly by gravity, with water often helping. It happens when the driving force (the weight of the material pulling it downhill) exceeds the resisting force (friction and cohesion holding it in place). Water adds weight and reduces friction between grains, which is why many landslides follow long rain.
- Soil creep: very slow movement, about 1 to 10 mm per year, shown by tilted fences and curved tree trunks.
- Rockfall: pieces fall freely from steep cliffs, building a scree slope at the base.
- Slump (rotational slide): a block of material slips along a curved surface, common in clay on cliffs.
- Mudflow and earthflow: saturated soil flows like a thick liquid.
- Landslide (translational slide): material moves along a flat surface, sometimes very quickly.
Triggers include heavy rain, earthquakes, volcanic eruptions, undercutting of slopes by rivers or waves, removal of vegetation and building on steep ground.
Types of mass movement compared
| Type | Speed | Water content | Typical sign |
|---|---|---|---|
| Soil creep | Very slow | Low to moderate | Tilted fence posts |
| Rockfall | Very fast | Low | Scree at cliff base |
| Slump | Slow to moderate | Moderate to high | Curved scar and stepped ground |
| Mudflow | Moderate to fast | High | Lobes of mud in valleys |
| Landslide | Fast | Variable | Bare scar on the slope |
Worked example: slopes, rates and load
- Slope angle: a hillside drops 30 m over a horizontal distance of 60 m. The slope is arctan(30 ÷ 60) = arctan(0.5) = 26.6°.
- Soil creep: at 5 mm per year, a fence post moves 5 × 100 = 500 mm = 0.5 m downhill in a century.
- Sediment load: a river carries 400 mg of sediment per liter and has a discharge of 50 m3/s. One cubic meter holds 1,000 L, so the load is 400 × 1,000 = 400,000 mg = 0.4 kg per m3. At 50 m3/s, this is 20 kg every second, or 20 × 86,400 = 1,728,000 kg = about 1,730 tonnes per day.
- Erosion rate of a catchment: if 1,000,000 tonnes of sediment leave a catchment of 500 km2 each year, the yield is 1,000,000 ÷ 500 = 2,000 tonnes per km2 per year. At a rock density of about 2.7 t/m3, this equals about 740 m3 per km2, or roughly 0.74 mm of lowering per year (simplified; soils are less dense).
- Stability: a block has a weight force of 1,000 N on a slope, with 400 N pulling it downhill. If friction can resist only 350 N, the net force is 50 N downhill, so the block moves. Heavy rain reduces friction and can tip the balance.
How did we learn about erosion?
In the 19th century, geologists such as Lyell and Hutton explained landscapes by slow, repeated erosion. G. K. Gilbert in the United States studied sediment transport in streams in the late 19th and early 20th centuries. Hjulström’s diagram (1935) related the speed of the water to particle size and showed whether grains are eroded, carried or deposited. The Dust Bowl of the 1930s in the United States, caused by drought and plowing of prairie grassland, led to national soil conservation programs. Modern studies use satellite images and laser scanning to map landslides and erosion.
Which mistakes are common?
Mistake 1: Treating erosion and weathering as the same
Weathering breaks rock in place. Erosion includes removal and transport.
Mistake 2: Thinking that mass movement needs water or wind
Gravity alone can cause rockfall and creep. Water makes slides more likely but is not always needed.
Mistake 3: Believing that rivers carry only what they can see
Much of the load is dissolved or in fine suspension. Bedload is only a small part of the total.
Mistake 4: Assuming that erosion only damages land
Erosion also supplies sediment for fertile floodplains, deltas and beaches. Problems arise when rates are far higher than natural ones.