Rivers in temperate ecoregions

– Water type description –

• Rivers in general

  A river forms a continuum, along which many parameters change: discharge, current, sediment and temperature, food sources as well as species composition. Small mountain rivers mainly have single channels, more downstream the channel changes to braided and meandering patterns. Floodplain and river are closely interlinked, for the main energy source for the macroinvertebrate and ultimately the fish community are leaves and wood.

  Like all other aquatic ecosystems in Europe river systems have been changed by a multitude of impacts. Small streams in mountain areas remained nevertheless relatively undisturbed; many small streams have never been severely polluted as almost all large rivers. Physical alterations in their catchments affect most European rivers, disrupt their continuum and the interactions between the stream and its terrestrial surroundings.

  Obviously, Climate Change will worsen this situation by increasing water temperatures and associated parameters. It will contribute to a general upstream movement of river zones, particularly affecting species bound to small streams and springs, which can not move further upstream. Most fish of small rivers, especially the salmonids, are cold-adapted and will be particularly affected by rising temperatures.

The pristine status

Rivers in temperature regions are characterised by permanent flow, but discharge may vary by orders of magnitude between seasons. Ice cover is an exception and mean summer temperature is a main factor for community composition. Species number is considerably higher than in cold ecoregions, but far lower than in the Mediterranean. Depending on size, altitude and catchment geology very different communities develop.

Human impact

While some near natural small stream can still be found, almost all medium-sized and large rivers have been deteriorated by a multitude of impacts. For decades organic pollution was most significant, while nowadays hydromorphological degradation (straightening, dams, and removal of riparian vegetation) is most widespread.

Climate change impact

Climate change will have some impacts on river hydromorphology, e.g. through more frequent floods, but will mainly effect river biota through increased temperatures. River temperature is a main determinant of community composition, and a general increase will lead to an upstream move of most communities and to a spreading of invasive species.