Temporary vs. Perennial Streams
Most large rivers are perennial, meaning they maintain flow throughout the year. However, many headwater streams or streams in arid regions sometimes run dry. A stream is considered temporary if surface flow ceases during dry periods. Temporary streams are often classified further as intermittent and ephemeral. An intermittent stream becomes seasonally dry when the groundwater table drops below the elevation of the streambed during dry periods. A spatially intermittent stream may maintain flow over some sections or surface water in deep pools even during dry periods due to locally elevated water tables or perched aquifers. An ephemeral stream only flows in direct response to precipitation such as thunderstorms. Thus, the flow variability of an intermittent stream is much more predictable than in an ephemeral stream.
In many parts of the world, such as the desert southwest, temporary streams may comprise a majority of the river network, >80% in some areas. However, even in wet regions, temporary streams at the head of river networks can account for >50% of the total stream network. Thus, river networks can be considered dynamic systems, with total miles of surface flow expanding and contracting in response to precipitation events.
Why would we still call a channel that goes dry for much of the year a stream? In other words, how can we distinguish between a temporary stream and an upland terrestrial ecosystem? In short, a stream has characteristic hydrological, geomorphological, and ecological processes. However, as with many topics in environmental science, the distinction between stream channels and uplands and between perennial streams and temporary streams is often fuzzy and scale-dependent. Individual stream channels may hold water for decades and then become dry during exceptional droughts that occur infrequently (once every 50-100 years). Similarly, small gullies on hillsides may flow only a few days of the year and may transport sediment but not be resident to aquatic life. Are such systems part of the river network?