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.