These aquifers are prevalent along present and ancestral stream valleys and in lowlands that are associated with structural or erosional basins. The unconsolidated deposits are mostly alluvial deposits, but in places, they consist of eolian, glacial, or volcanic deposits. Alluvial deposits consist primarily of well-sorted particles that range in size from clay to boulders. Eolian deposits, or loess, consist chiefly of clay, silt, and fine sand. Although loess is well sorted, it does not form productive aquifers because it is fine grained, usually unsaturated, and commonly is only a veneer overlying other rocks.
Glacial deposits consist chiefly of mixtures of particles that range in size from clay to boulders.
Where these deposits are well sorted, they form productive aquifers. Volcanic deposits consist chiefly of ash and basaltic sand, particularly in southwestern Idaho and southeastern Oregon. These deposits, which commonly are interbedded with thin flows of basalt and welded tuff, generally have minimal permeability and do not form productive aquifers. Typically, unconsolidated deposits along stream valleys consist chiefly of sand and gravel that form productive aquifers. The thickness of the deposits along present stream valleys commonly is less than 250 feet.