ALLUVIUM, soil or land made up of the sediment deposited by running water. Rivers act on the rocks in their course both mechanically and chemically, and are in consequence always more or less loaded with detritus, which in its turn again aids the water in abrading other rocks. A great proportion of the matter with which rivers are thus charged is carried out to sea. But in level tracts, where the motion of a river is slow, it frequently overflows its banks, and leaves a sediment of earth, mud, gravel, &c., when it returns to its ordinary channel. The principal alluvial tracts are the deltas or deltoid formations at the mouths of large rivers. These vary in character very considerably. The Delta of the Nile is the best-marked specimen; the waters of the Rhine, Ganges, &c., arrested by the solid matter they have washed down, force their way through it in numerous smaller channels; the Mississippi has carried the solid matter it holds in suspension far into the Gulf of Mexico, forming long spits of land on the banks of the stream. The cognate term diluvium (now little used) has been applied to formations produced by extraordinary aqueous agencies.