A general term for a body of flowing water; natural water course containing water at least part of the year. In Hydrology, the term is generally applied to the water flowing in a natural channel as distinct from a canal. More generally, as in the term Stream Gaging, it is applied to the water flowing in any channel, natural or artificial. Some classifications of streams include, in relation to time:
 Ephemeral Streams—Streams which flow only in direct response to precipitation and whose channel is at all times above the water table.
 Intermittent or Seasonal Streams—Streams which flow only at certain times of the year when it receives water from springs, rainfall, or from surface sources such as melting snow.
 Perennial Streams—Streams which flow continuously.
 Gaining Streams—Streams or a reach of a stream that receive water from the zone of saturation. Also referred to as an Effluent Stream.
 Insulated Streams—Streams or a reach of a stream that neither contribute water to the zone of saturation nor receive water from it. Such streams are separated from the zones of saturation by an impermeable bed.
 Losing Streams—Streams or a reach of a stream that contribute water to the zone of saturation. Also referred to as an Influent Stream.
 Perched Streams—Perched streams are either losing streams or insulated streams that are separated from the underlying ground water by a zone of aeration.