Between 2000 to 2002, 11 experimental floods were released from 2 reservoirs separating different regulated segments of the Spöl River in the Swiss National Park. The riverbed of the Spöl had been altered substantially by regulated flows since energy production started in 1970. This 3-year experiment aimed to test whether an annual flood regime could restore riverine dynamics to this regulated river and, if so, how the floods must be managed in respect to a minimal but regular flow disturbance required to maintain riverine dynamics in a near natural state. This article outlines the framework, objectives and design of the experimental flood program on Aquatic Sciencesthe River Spöl. The study involved a number of institutions controlling operational, economical, legal and ecological matters related to the floods, and was planned under the constraint that the floods would result in no increase in total water release during each year and no loss in total energy production. Concomitant cross-disciplinary environmental studies monitored the diverse impacts of each flood on the respective regulated segments. The various results, as shown in the accompanying reports, suggest that one or two annual high flows may be sufficient to enhance and sustain the ecological integrity of the Spöl over the long term.

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

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.

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