Temperate temporary running watercourse

This habitat type includes temporary freshwater streams and rivers in the European temperate region. These water bodies are characterized by strong fluctuations in water level, which includes dry periods, alternating with long periods of running water. These fluctuations in water level are related to the level of the underlying water table and the amount of precipitation. In the United Kingdom the term ‘chalk streams’ is used to describe watercourses developing on chalk rock formations. This very soft and porous geological substrate acts as temporary reservoir. More than 80 % of the annual stream discharge originates from the aquifer in these chalk-based systems. The slow release of water from the aquifer provides a relatively stable hydrological regime despite the concentration of rainfalls in some seasons. Chalk streams can be subdivided into different hydrological categories. Only the winterbourne ones have a natural dry period each year and are considered part of the habitat C2.5a Temperate, temporary running waters. Chalk streams and rivers that never dry out are not included in this habitat. The seasonal cycle of wetting and drying results into characteristic plant communities that have adapted to these situation. The main channels are often dominated in spring by aquatic Ranunculus beds, consisting mainly of Ranunculus peltatus or R. penicillatus subsp. pseudofluitans. Note that Ranunculus fluitans is not typical of this habitat, because require more stable water level. Grasses and herbs are dominating the shores and accompanying marshes. Those include a number of annual species that appear in autumn after re-wetting of the shores. These temporary streams differ from Mediterranean temporary rivers mainly for the duration of the completely dry period (that is shorter) and for the absence of clearly Mediterranean floristic elements. If the dry period is prolonged and the hydrologic conditions very irregular these temporary streams could also scarcely vegetated.

England is usually considered to have the major part of properly defined ‘chalk streams’ of Europe. They are located in and down-stream of areas of outcropping chalk. It is unclear which other countries have similar streams, but surely temporary streams with similar hydrology they occur also in other karst areas of Europe and on substrates averagely permeable such as turbidite deposits rich in calcareous elements. In former times in United Kingdom, ‘chalk streams’ were connected to floodplains and wet meadows, representing systems with a high biodiversity and luxurious plant growth. Nowadays they are generally highly modified systems.

Indicators of good quality:

• The periodical alternation of wet-and-dry regimes
• The absence of heavy anthropogenic activities that can alter the hydrogeological system (e.g. water capitation and drainage, artificial shores for flood defence)
• The absence of communities and species indicating an excessive nitrification or disturbance such as ruderal and exotic species
• Water course connected to floodplains and wet meadows

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats