Perennial stream

A perennial stream or perennial river is a stream or river (channel) which always flows in some parts all year round during years of normal rainfall. Perennial streams are different to intermittent streams; which normally stop flowing for weeks or months each year. They are also different to ephemeral streams that flow only for hours or days after rainfall. During unusually dry years a perennial stream may stop flowing. In a drought, they may not flow for months. The differences between perennial, intermittent, and ephemeral streams are not exact and can change if different measures are used by different groups.
In dry weather it may not be possible to see the water flowing, or see any water at all. In streams with gravel, sandy or rock bottoms, the water may be flowing beneath or between the rocks. In biology, a stream is flowing if there is enough water to support aquatic life, including fish and gill-breathing amphibians, benthic insects, crustaceans, and mollusks. These can live in a shallow flow beneath rocks or logs. The flow may be so small that it can not be measured by stream flow measurement instruments.