Following the construction of two large reservoirs in the late 1960s on the River Spöl, Swiss National Park, flow was greatly reduced and regulated at a relatively constant discharge. The regulated flow regime resulted in the elimination of river changing floods, causing altered and degraded habitat conditions for the brown trout (Salmo trutta fario L.). Although food resources (i.e., zoobenthos abundance) increased after flow regulation, trout spawning areas were greatly reduced by the clogging of coarse sediments. Consequently, the National Park in agreement with power authorities initiated an Aquatic Sciences experimental flood program in 2000 to improve the fisheries potential of the River Spöl. Fish abundance was not reduced by the floods and relatively few fish (< 2 %) were killed or stranded by the floods. In fact, the quality of fish habitat, spawning grounds in particular, was noticeable improved, even though food resources were altered to some degree by the floods. The results showed that the condition of trout in the Spöl remained relatively constant, but the number of redds has increased three-fold since initiation of the flood program.

After the construction of two relatively large reservoirs in the late 1960s on the River Spöl, discharge was greatly reduced and regulated at a relatively constant flow. Following flow regulation in 1970, natural floods occurred only in the lower Spöl where the river is joined by a large tributary, the River Cluozza. Flow competence of the residual flow in the regulated river section was too low to transport downstream the input of inorganic and organic sediments from tributaries and side-valley scree (talus) slopes. Consequently, sediments accumulated on the riverbed, and alluvial fans from scree slopes extended Aquatic Sciencesinto the river channel. The lack of flood disturbance also allowed woody vegetation to develop on previously exposed gravel banks, and the porous river bottom became clogged with fines. After the experimental floods in 2000, most alluvial fans in the channel were scoured downstream and bed sediments became less embedded due to the reduction in fines. The initial floods caused a rather broad accumulation of coarse sediments in wider reaches of the river, whereas the later floods mobilized/scoured these sediments and increased the variation in channel depth.

Le régime hydrologique naturel de la grande majorité des rivières du globe a été modifié par une régulation artificielle, qui a profondément affecté la morphologie fluviale et la vie aquatique. L’intégration de critères hydrologiques comme le débit et la température dans les programmes de restauration constitue une étape importante pour la gestion de rivière. Cet article synthétise les observations, en terme de qualité physicochimique de l’eau et de biocénose aquatique, des effets d’une programmation de crues sur le long terme (15 crues artificielles en huit ans) sur la rivière Spöl, dans le Parc National Suisse. Du fait des lâchers d’eau hypolimnétiques (issues des eaux profondes), ces crues ont peu d’impact sur les paramètres physiques et chimiques. La biomasse du périphyton a été réduite par les premières crues, puis s’est maintenue à des niveaux faibles pendant toute la période étudiée. La richesse spécifique, la biomasse et la densité de macro-invertébrés ont aussi été significativement réduites, et l’association de macroinvertébrés a évolué vers des taxons plus résistants aux perturbations. La qualité des habitats piscicoles, en particulier pour les zones de frai, a été sensiblement améliorée par les inondations. Une analyse plus approfondie a montré que la réponse de la biocénose à des crues d’ampleur similaire a changé pendant la période d’étude en parallèle avec la modification de la composition des associations biotiques.
The natural flow regime of many rivers on the globe has been altered by regulation, strongly influencing river morphology and aquatic biota. The incorporation of regimebased criteria such as flow and temperature regimes in restoration plans is an important step in river management. This paper summarizes the effects of a long-term flood program (15 floods over 8 years) on the river Spöl, Swiss National Park, on water physico-chemistry and river biota (periphyton, macroinvertebrates and fish). Due to hypolimnetic release, the floods had little impact on physical and chemical parameters. Periphyton biomass was reduced by the early floods and remained at low levels throughout the study period. Macroinvertebrate taxon richness, biomass and density were also significantly reduced and the macroinvertebrate assemblage shifted towards more disturbance-resistant taxa. The quality of fish habitat, especially for spawning, was noticeably improved by the floods. Further analysis suggested that the response of biota to floods of similar magnitude have changed over the study period in concert with the compositional shift in biotic assemblages

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Hydropower is usually considered a relatively "green" energy; it is renewable and does not cause emissions of carbon dioxide or other pollutants. However, outflows of dams often cannot be considered ecologically sound rivers; they are not polluted but altered in their physical regime, mainly through changes in the temporal dynamics of flow and temperature. During the last decade a paradigm shift has occurred in the way river managers perceive how flowing waters should be managed and restored. Regime-based river management is gaining momentum. Floods are an important natural disturbance, influencing river structure and functioning. In the Spöl River, Switzerland, a flow-regime protocol using experimental floods has been implemented toward restoring river integrity downstream of a large dam. The results suggest that regime-based management has high potential for improving ecosystem integrity while meeting human needs such as power production.