Brian Doran, age 15, of Burlington, Vt., for his question:

WHAT IS ALLUVIUM?

Alluvium is silt, sand, gravel or similar loose material that is deposited by flowing water. You'll find that alluvium usually occurs at any point where the velocity of fast running water is suddenly slowed and the carrying capacity of the flow is reduced to a point where transport of the sediment is no longer possible.

Alluvium deposits are found on the flood plains of river valleys, through river deltas and where steep mountain streams enter quiet lakes or flow out on a comparatively flat surface.

When the deposit, in the course of its accumulation, takes the shape of a fan or cone, as it often does at the base of a mountain, it is then called an alluvial fan.

Among the great alluvial deposits of the world are the deltas of the Nile in Egypt, the Hwang Ho or Yellow River in China and the Ganges in India.

Another of the great alluvial deposits is the flood plain of the giant Mississippi River where it empties into the Gulf of Mexico.