Hydrological regime modification

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Hydrological regime modification

02. Flow regulations

General description

The impacts caused by dams and reservoirs in rivers is a problem of global nature that affects the major rivers around the world (Petts, 1984; Dynesius & Nilsson, 1994; Jalon Garcia et al., 1992, Nilsson & Berggren, 2000; etc.)..

Regarding irrigation reservoirs in Mediterranean rivers, first there is the very significant transformation of the natural flow regime into a regulated regime in opposed phase: minimum monthly flows that naturally occur in summer, are increased into maximum ones due to the demand for water for irrigation, while the minimum flows occur in the winter months. colder and more precipitation, which tries to fill the reservoirs (see Figure 1).

Figure 1.- Flow Regime river Porma (Spain) affected by the dam Juan Benet (irrigation reservoir), distinguishing two periods, an unregulated (1942-1968) and other post-exploitation of the dam (1969-2004).







With the filling of reservoirs during the months of rain (winter) are eliminated or ordinary floods of higher frequency, which are of great importance in maintaining the natural channel morphology and the renovation of habitat and regeneration of the riparian vegetation, and generating a flow regime much more uniform in time, where only keep the avenues overtime (Figure 2).

The transformation of the natural flow generated from the regulation for irrigation depends on the location on the river network of the stretch of river in question. It can only mean a change of the times in which they occur peak flows, or also include a drastic reduction of water flowing through the channels with respect to the contribution natural, downstream from the derivations of water for use in agriculture.


Figure 2.- River Aragon flow regime, downstream of the dam of Yesa. Since 1970 at which the regulation for irrigation significantly reduces the frequency of regular floods, between 100 and 200 m3/s, and increases the length of minimum flows, so will these impacts over time.



The regulation of the flow affects very significantly to the physical habitat of the rivers where they live aquatic communities of plants and animals, or those associated with moisture inland. The affections to the aquatic habitat may refer to the emergence of hydraulic conditions relating to the depth of the water, speed of current, shear stress,.., unsuitable for native species and may affect a mismatch of spawning for fish reproduction, lack of deep areas of refuge for individuals larger, and so on. They can also alter the physical and chemical characteristics of water, with reduced forms of dissolved salts, problems of dissolved oxygen content, or altering the temperature regime in connection with fund outflows of reservoirs, with summer temperatures in much colder that the air which impacts on the development of biological cycles of aquatic insects.

In the stretches of rivers are also regulated extensions or modify the surface useful for each species, to vary the area of film and the distribution of water depths in each section of the channel depending on the flow circulating. It also changes the size and arrangement of sediment to the bed and banks; when traveling on a prolonged the minimum flows (during autumn and winter) is the siltation of the bed by accumulation of thin, with a seal of microhabitats, while when traveling on a continuous peak flows (months irrigation) increases the drag forces on the bed, causing instability in the macro, and how the banks are a size too thick to retain moisture and germination and growth of seeds.

Dams are barriers that inhibit fish migrations, but their effect on the river continuum is much more important because affects all its components though the fragmentation of all the fluvial network (Dynesius & Nilsson, 1994).

Some of these effects result immediate from the starting of the reservoir regulation and others show a delay in their appearance, especially the changes in the composition and structure of communities biological. In general, new water controlled conditions are less favorable to native species adapted to a natural with Mediterranean avenues regular season marked a certain magnitude and in the warmer months, and more favorable for exotic species from other regions They can be more competitive than the first in the schemes covered more uniform, similar to those of the rivers of origin.

This irrigation regulation produces impacts that affect in more or less degree the aquatic system and the dependant environments. Changes in the river flows (in terms of quantity, timing, and frequency) can lead directly to biologic and geomorphic changes, and geomorphic changes lead indirectly to ecological adjustments (Whiting, 2002), altering ecosystem stability in terms of resistance and resilience (Connel & Wayne, 1983). As some authors have mentioned, under regulated conditions Mediterranean stream species cannot compete successfully with many introduced (generalist and limnethic) species (García de Jalón et al., 1992; Morillo et al., 2002), diminishing the ability of aquatic systems to maintain a balanced, integrated, and adaptable community of autochthonous organisms.

The effects of regulating the flow into the fish species have been studied in depth in Spain and are well documented in different jobs (Elvira, 1995; Elvira and Almodovar, 2001, etc.). It is clear that the maintenance of regulated flow regimes, with neither annual or interannual fluctuations, including bankfull discharges floods for natural regeneration of the river habitat, has created a different fluvial habitat that has favor a strong invasion of mediterranean rivers for many species, both fish (e.g., catfish, sun-fish, carp, pike, salmon of the Danube, etc.). as other animal species (e.g., zebra mussel) or aquatic plants (e.g. water hyacinth).

Mediterranean streams have natural regimes with an important torrential component that is reflected in strong seasonal and interannual fluctuations. At evolutionary scale, the geological and biogeographical history of these rivers (together with these particular hydrological features), has produced a large number of endemic species since many fluvial basins have remained isolated for a long time. As a consequence, the degree of endemicism of primary and secondary freshwater fishes in Spain is remarkable (Elvira, 1995). These species are not adapted to the new habitats and cannot compite successfully with introduced ones.

Effect/Impact on (including literature citations)

  • HYMO (general and specified per HYMO element)
  • physico - chemical parameters
  • Biota (general and specified per Biological quality elements)

Case studies where this pressure is present

Possible restoration, rehabilitation and mitigation measures

Useful references

Dynesius, M. y C. Nilsson. 1994. Fragmentation and flow regulation of river systems in the northern third of the world. Science, 266: 753-762. García de Jalón, D., M. González del Tánago y C. Casado. 1992. Ecology of regulated rivers in Spain: An overview. Limnetica, 8: 161-166. González del Tánago, M. 1996. Impacto de la agricultura en los sistemas fluviales. Técnicas de restauración para la conservación del suelo y del agua. Agricultura y Sociedad, 78: 211-236. Kriakeas, S.A. y M.C. Watzin. 2006. Effects of adjacent agricultural activities and watershed characteristics on stream macroinvertebrate communities. Journal of the American Water Resources Association, 42(2): 425-441. MAPA (Ministerio de Agricultura, Pesca y Alimentación) 2003. El Libro Blanco de la Agricultura y el Desarrollo Rural. Publ. Ministerio de Agricultura, Pesca y Alimentación, Madrid. MMA (Ministerio de Medio Ambiente). 1998. Libro Blanco del Agua. Ministerio de Medio Ambiente, Madrid. Morillo, M., A. Gimenez and D. Garcia de Jalón. 2002. “Evolución de las poblaciones piscícolas del río Manzanares aguas abajo del embalse de El Pardo (Madrid).” Limnetica, 17: 13–26. Nilsson, C. y K. Berggren. 2000. Alterations of Riparian Ecosystems caused by River regulation. BioScience, 50(9): 783-792. Petts, G.E. 1984. Impounded Rivers. John Wiley & sons, Chichester.

Other relevant information

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