Implementing the Water Framework Directive to temporary rivers

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From flowing to dry conditions: first results on the aquatic states

During the last 6 months we have been visiting our study sites regularly (every six weeks) in order to capture the different hydrological conditions and aquatic states of streams from spring to end of summer, and sample the aquatic communities living therein. We observed that many temporary streams with abundant flowing water in spring (Eurheic state) progressively dried out, passing through a stage with a succession of pools connected by a tiny river flow (Oligorheic state), disconnected pools (Arheic state) and, finally, until they got completely dry (Hyporheic state) . As shown in the maps, this included natural temporary streams, such as Talamanca, Pineda, Monlleó and Cérvol, but also permanent ones affected by water abstraction, such as Guadazaón. Other streams did not dry out and kept disconnected pools all along the summer, and reconnected with the first rains in September. This was the case of the Glorieta and Tossa streams, and the Matarranya River.

Click on the dots to obtain more information: to know the river’s name and its hydrological flow regime, and see a picture of it.



FiguraEstats Aquàtics

Brief description of the different aquatic states that a temporary river may go through. Figure: N.Cid.

Some streams that in natural conditions would be temporary (intermittent, seasonal or ephemeral) suffered from “perennialization” mainly due to the effluents from the sewage treatment plants. That is, water flowing continuously where it’s not supposed to. These changes alter the natural temporary water regime of the river, and, in most cases, contribute to the impoverishment of biological communities due to pollution. This was the case of Torrent del Puig, Congost and San Miguel Rivers.

Ephemeral streams did not have water at any time during our visits and therefore they presented a Hyporheic state during all the sampling period. Flow in these systems is only originated during heavy rains of autumn or spring and therefore it is characterized by pulses, which did not occur during our samplings. None of our streams was sampled under a hyperrheic state.

In contrast, permanent streams with no significant hydrological impacts, such as Aigua d’Ora and Algars, and Cabriel, only the Eurheic and Oligorheic states were observed.

Now we are waiting to come back again when all our temporary streams will be rewetted, to be able to record duration of the Hyporrheic state. Once biological data will be processed, this will help us analysing how communities respond to different severity levels of flow intermittency.

Related articles: World Rivers Day: raising awarenes for the importance of temporary rivers