Silt is very fine particles of rock and soil, smaller than sand but larger than clay, that water carries and deposits. When rivers flood or lakes dry up, they often leave behind layers of silt that feel smooth and soft, almost like flour or talc powder.

Rivers are constantly moving silt from higher ground toward the ocean. As the water slows down, especially where rivers meet the sea or spread out across floodplains, the silt settles to the bottom. Over thousands of years, this process built some of the world's richest farmland, like Egypt's Nile River valley or Louisiana's Mississippi Delta. Ancient civilizations flourished in these places partly because seasonal floods deposited fresh silt that made the soil very fertile.

Silt can also cause problems. Harbors and irrigation channels gradually silt up as particles accumulate, requiring dredging to clear them out. Too much silt washing into a lake or bay can cloud the water and harm fish and plants.

When you walk barefoot along a calm riverbank or lakeshore, that soft, silky mud squishing between your toes is probably silt. It's one of the things that can make certain kinds of mud good for making pottery, since it holds together well but usually isn't as sticky or heavy as pure clay.