The soil, clay, silt, or gravel that is left by a flowing water surface which goes down and away from a mountainside and merges with a plain or wide valley floor: Alluvial slopes are made up of the surface soil from hills or mountains that have been "washed" by rain during which time the slanting hills deposit bushes and other vegetable matter, plus sand, dirt, and mud slides, all of which are carried down and left in areas of flat land, such as valleys.