Intermittent river


Intermittent rivers cease to flow every year or at least twice every five years. Such rivers drain large arid and semi-arid areas, covering approximately a third of the earth’s surface. The extent of temporary rivers is increasing, as many formerly perennial rivers are becoming temporary because of increasing water demand, particularly for irrigation. Despite inconsistent water flow, intermittent rivers are considered land-forming agents in arid regions, as they are agents of significant deposition and erosion during flood events. The combination of dry crusted soils and the highly erosive energy of the rain cause sediment resuspension and transport to the coastal areas. They are among the aquatic habitats most altered by human activities. During the summer even under no flow conditions the point sources are still active such as the wastewater effluents, resulting in nutrients and organic pollutants accumulating in the sediment. Sediment operates as a pollution inventory and pollutants are moved to the next basin with the first flush. Their vulnerability is intensified by the conflict between water use demand and aquatic ecosystem conservation. Advanced modelling tools have been developed to better describe intermittent flow dynamic changes such as the tempQsim model.

Categorization and Distribution

An intermittent river, or intermittent stream, is any river or stream that only flows during certain times of the year, and may not have any flowing surface water during the dry season. Intermittent rivers do not rely on, but may be supplemented by stormwaters or other runoff from upstream sources. Their channels are well-defined, as compared to ephemeral streams, which have no defined channel, and rely mainly on storm runoff. Intermittent rivers are found on every continent, and may even be more common than perennial rivers. More than 30% of the total length and discharge of the global river network is estimated to be intermittent rivers. However, due to some low-order streams being difficult to categorize or track, this total could be over 50% when taking those into account. In the face of global climate change, this total is further increasing, as many of the world’s river that were once perennial are now intermittent in regions suffering from severe climatic drying or water appropriation.

Ecology

The inhabitants of intermittent rivers can change with the water level. As a result of contrasting conditions throughout the year, invertebrate assemblages of the same intermittent stream can be notably distinct from one another. How biodiversity of these habitats changes with conditions has been debated in literature. Current findings suggest that while lotic biodiversity generally decreases with increasing flow intermittence, increased lentic and terrestrial biodiversity during those periods can compensate. Thus, when lotic, lentic, and terrestrial communities are considered together, intermittent rivers can account for a high proportion of regional biodiversity. The riparian zone of intermittent rivers can provide habitat and resources for a variety of organisms, and may also be an important source of nutrients for habitats downstream.