A general term for a body of ﬂowing water. In hydrology, the term is generally applied to the water ﬂowing in a natural channel as distinct from a canal. More generally, it is applied to the water ﬂowing in any channel, natural or artiﬁcial. Some types of streams include: (1) Ephemeral: a stream that ﬂows only in direct response to precipitation, and whose channel is at all times above the water table; (2) Intermittent or Seasonal: a stream that ﬂows only at certain times of the year when it receives water from springs, rainfall, or from surface sources such as melting snow; (3) Perennial: a stream that ﬂows continuously; (4) Gaining: a stream or reach of a stream that receives water from the zone of saturation. It is an efﬂuent stream; (5) Insulated: a stream or reach of a stream that neither contributes water to the zone of saturation nor receives water from it. It is separated from the zones of saturation by an impermeable bed; (6) Losing: a stream or reach of a stream that contributes water to the zone of saturation. An inﬂuent stream; (7) Perched: a perched stream is either a losing stream or an insulated stream that is separated from the underlying groundwater by a zone of aeration.