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Pour water on a slope of sand: where does the sand go?
Erosion lab · Last updated: 16 October 2026
You pour water down a slope of sand and watch what happens: channels form, grains wash away and new land builds up where the water slows down. The same process, over thousands of years, carves valleys and builds the deltas where great rivers meet the sea.
What you'll discover
- Moving water picks up loose grains and carries them downhill, wearing the land away.
- Faster water carries more and bigger grains, so steep slopes erode more quickly.
- When the water slows down it drops its load, building fans and deltas lower down.
The science in plain words
Erosion is the removal and carrying away of rock and soil by water, wind, ice or gravity. It usually follows weathering, which breaks rock into smaller pieces. Running water is the most powerful agent of erosion on land.
When water flows over loose sand, it drags grains along. How much it can carry depends on its speed: faster water lifts more grains and larger ones. On a steep slope water speeds up, so erosion is strongest there. Once a small channel forms, more water collects in it and flows faster, digging it deeper. This is how tiny gullies grow into streams and, over very long times, into river valleys.
Whatever the water picks up, it must put down somewhere. When the slope flattens or the water spreads out, it slows down and drops its load, the heaviest grains first and the finest mud last. This is called deposition. At the foot of mountains it builds fan-shaped deposits, and where rivers enter the sea it builds deltas, such as the Ganges and Brahmaputra delta, the largest in the world. Plant roots hold soil together and leaves soften the impact of rain, which is why bare hillsides and fields lose soil much faster than those covered in plants.
Where it fits
- India, NCERT: Erosion is not a chapter of the NCERT science syllabus for classes 6 to 10; it is taught in NCERT geography.
- United States: Grades 3 to 5
- Next Generation Science Standards: 4-ESS2-1
Common questions
What is the difference between weathering and erosion?
Weathering breaks rocks down into smaller pieces where they are, through rain, frost, heat, plants or chemical change. Erosion moves those pieces away, carried by water, wind, ice or gravity. Weathering prepares the material and erosion transports it somewhere else.
How do plants stop soil erosion?
Plant roots bind soil particles together, and leaves and fallen plant matter shield the ground from heavy raindrops. Plants also slow down water running over the surface. That is why farmers plant cover crops and why hillsides stripped of trees often suffer landslides and heavy soil loss.
How do rivers make deltas?
A river carries sand and mud as it flows. When it reaches the sea or a lake, it slows down and can no longer carry its load, so the sediment settles. Over thousands of years this builds new land that spreads out in a fan shape called a delta.
Related labs
- Rock Cycle: Follow a grain of rock: worn away, buried, cooked, melted and born again.
- Water Cycle: The Sun lifts water out of the sea. Where does it go, and how does it get back?
- Plate Tectonics: The ground moves as fast as your fingernails grow. What does that do in a million years?
See all earth and space labs or every Taz science lab.
Try it yourself
The Erosion lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.