alluvium, type of sediment which can be described as alluvial, giving rise to soils which are often fine grained and typically fertile consisting of mud, silt, sand, and sometimes gravel or stones deposited by flowing water on flood plains, in river beds, in deltas, and in estuaries, often from many different and distant sources. Alluvial soils are variable in texture, drainage, and age, and often such changes can be seen over a few metres. Where these soils are stony and sandy, they are highly valued for viticulture, as in the médoc region of France and Marlborough in new zealand. See entries prefixed soil.
