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Deposition is what happens when erosion stops — when moving water, wind, or ice slows down enough that it can no longer carry its sediment and drops it. The deposited material builds new landforms: rivers create deltas and floodplains, wind creates sand dunes, glaciers leave behind moraines, and ocean waves build beaches. Deposition is the flip side of erosion — erosion takes material away from one place and deposition puts it down in another. The heaviest particles are deposited first (when the carrier slows just a little) and the lightest particles last (when it stops completely).
Use a stream table to show how a river deposits material when it reaches flat ground or enters a body of water — watch a delta form in real time. Pour a mixture of gravel, sand, and clay into a jar of water and observe how particles settle by size — heaviest first, lightest last. Compare satellite photos of river deltas (Mississippi, Nile) and glacial moraines. Build a sand dune using a fan — watch how the dune migrates downwind as sand is carried up the windward side and falls down the lee side.
Erosion and deposition are two halves of the same process. Erosion picks material up from one place; deposition sets it down in another. Wherever an erosive force — water, wind, or ice — slows down or stops, it drops the sediment it has been carrying, and a new landform begins to grow.
The most familiar depositional landform is a river delta. When a river flows from mountains to the ocean, it carries sand, silt, and clay in its current. The moment it reaches the ocean and stops flowing, it drops that sediment. Over centuries and millennia, the sediment piles up into a fan-shaped deposit that extends into the water. The Nile Delta in Egypt and the Mississippi Delta in Louisiana are enormous examples — built grain by grain from material eroded hundreds or thousands of kilometers upstream. Deltas are some of the most fertile land on Earth because they contain nutrient-rich sediment collected from across entire river basins.
Rivers also create floodplains — flat areas alongside the river channel that get covered with sediment during floods. When a river overflows its banks, the water spreads out and slows down, depositing fine mud and silt across the flat land. This is why floodplain soils are so fertile and why farmers throughout history have built their fields along rivers, despite the flood risk.
Wind creates its own depositional landforms. When wind carrying sand encounters an obstacle or slows down, the sand accumulates into dunes. Dunes are not static — they migrate slowly downwind as sand is blown up the gentle windward slope and tumbles down the steep leeward slope. The Sahara Desert contains dunes hundreds of meters tall, built entirely from wind-deposited sand.
Glaciers leave behind moraines — ridges and mounds of rock, gravel, sand, and clay that were pushed forward or carried along by the ice and dumped when the glacier melted. Terminal moraines mark the farthest point a glacier reached; lateral moraines line the valley sides. The rolling, hilly terrain of the northern United States and much of Europe is glacial deposit landscape — material carried south by ice sheets during the last ice age and left behind when the ice melted about 10,000-20,000 years ago.
A key principle of deposition is sorting: when a carrying agent slows down, it deposits the heaviest particles first and the lightest last. This is why you find boulders at the mouths of mountain canyons, sand farther out on the plain, and only the finest silt and clay reaching the ocean. This sorting is preserved in sedimentary rocks and gives geologists clues about the ancient environments where those rocks formed.