Alluvium is the soil itself — the sand, silt, clay, and small stones that rivers, lakes, and floods leave behind. After a flood, the new layer on the fields is alluvium. Over many years it can grow into a thick bed. This is a geography and geology word. In ordinary talk, people say mud, silt, or “the soil the river left.” Gold and other minerals sometimes sit in alluvium, which is why you hear of alluvial gold: gold that water has washed down and dropped, rather than gold still locked in rock.