River researchers have forgotten temporary rivers since many years. In front to large, tropical or temperate rivers, temporary rivers carry a very low amount of water to the oceans. In most of them there are no long-term flow records. They are usually known for both, the disaster that these rivers originate from time to time due to large precipitation events and, for just the contrary, the dryness of their riverbeds for a large part of the year. Many of these rivers are located in Mediterranean climate areas or semi-arid areas of the world, just where water scarcity threatens human civilizations. Some temporary rivers are the result of dramatic water abstraction and other just receive raw sewage or sewage waters coming from water treatment plants.

However, temporary rivers hold an enormous variety of life, sometimes with incredible life histories. One amazing example is found in some Trichoptera species whose adults aestivate in caves when rivers are dried in summer, and return to them with the first autumn rains. Another incredible example can be found in several species that have strategies to survive in the dry riverbeds and reappear also with the first rains. Or the unique crabs and snails found in permanent pools in Australia that may remain isolated several years from the river. In some cases temporary rivers may be more diverse that permanent ones, because they contain different communities along a year in accordance to the hydrological cycle.

The cycle in a temporary river begins with the initial flooding just after the autumn rains (hyperheic state). After these floods, temporary rivers look like permanent streams, with sequences of riffles and pools, and may even hold a similar fauna (eurheic state). At the end of spring, river flow decreases and riverbeds can have a succession of pools connected by a tiny river flow (oligorheic state). When the flow stops the river is formed by several isolated pools, of different size and volume (arheic state) as you can see in the picture.

With the dominance of pools, the community changes completely and the bugs present in these pools are totally different from those found in spring. The pools may remain along summer until autumn when the flow returns, or may dry out, either with yet some aquatic fauna living buried in the wet alluvium (hyporheic state) or the dry riverbed being colonized by terrestrial invertebrates (edaphic state). At the end, the number of taxa along the year is higher than in permanent rivers. Thus, the hydrological conditions and biological communities in temporary rivers change with time but can be also very variable in space.

Different conditions can be found simultaneously in the same river network, indicating that temporary rivers are spatiotemporal shifting mosaics. Depending on the size of the pools, temperature or other factors, a temporary river may hold many different communities at the same time: permanent in the headwaters, intermittent in the mid parts and ephemeral in the lower parts (or vice-versa). All this mosaic may disappear due to human impacts (e.g. water abstraction, pollution), putting the temporary rivers and its splendid biodiversity at high risk.

In the next years, and due to global change, many permanent rivers may become temporal. Therefore, understanding how temporary rivers need to be preserved and managed is of vital urgent importance, especially in Mediterranean climates where temporary rivers are part of our landscape. This is the objective of the project TRIVERS a LIFE+ project that is currently going on and studying temporary rivers in Eastern Spain.

In occasion of the World Rivers Day (worldriversday.com) we would like to send a message to river scientists and citizens of the world, to value and take care of such rivers to halt biodiversity loss: temporary rivers may be as beautiful as permanent ones and their sustainable use may provide important ecosystem functions and services to the inhabitants of large part of our world.