Dunes form where loose sediment accumulates. Requirements for dune formation are:

1. Loose sediment, usually sand-sized. Environments with abundant loose sediment include beaches, river bars, and deserts.
2. Energy to move sediment, usually wind, but dunes can form underwater in rivers.
3. Some obstacle around which sand accumulates. Areas with sparse vegetation commonly form dunes. Dunes also form around rocks.
4. A dry climate. Moisture causes sand grains to stick together. Larger grains are more difficult to transport by wind. Moisture also facilitates plant growth, which stabilizes loose sediment and prevents it from moving.