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.

The river Spöl flows from Livigno reservoir on the Swiss-Italian border in the central Alps. Flow regulation since 1970 has decreased and stabilized the mean annual discharge of 8.6 to 1.0 m 3 /s (winter: 0.55 m 3 /s, summer: 2.5 m 3 /s). Thirty experimental floods were released between 2000 and 2016 to improve the ecology of the river. A 100-m reach~2.3 km downstream of the dam was used to monitor changes in water physicochemistry, periphyton, transported and benthic organic matter, and macroinvertebrates. Fish redds (native Brown Trout) were counted each autumn along the same stretch (~2.6 km) of river. Most physicochemical variables displayed strong seasonal cycles unrelated to the flow program, but N, P, and water temperature increased significantly over the 18-y study. Seston decreased after each high flow, but was seasonal and increased over the study. Periphyton biomass decreased after each high flow to low levels and increased between high flows. Benthic organic matter decreased 2Â over the study but increased between floods. Macroinvertebrate densities decreased over the study. High flows reduced macroinvertebrate densities to <10,000 individuals (ind)/m 2 , with densities usually increasing to~10,000 to 20,000 ind/m 2 between floods. Floods reduced taxonomic richness with increases between floods. Ordination showed temporal shifts in macroinvertebrate assemblages over the study. Mayflies and stoneflies responded positively and chironomids and gammarids negatively to the floods over the study. Fish redds increased from 58 in 1999 to >200 by 2003. Redd counts decreased to <30 after an accidental sediment spill in 2013 but rebounded to 153 redds in 2016. Macroinvertebrate assemblages recovered within months after the spill. Long-term monitoring was essential for elucidating ecosystem dynamics and evaluating biological responses to pulse disturbances, such as the sediment spill, during the study.

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

The incubation period represents an important development phase for successful reproduction of gravel-spawning fish, whereby colmation processes can affect the quality of the interstitial habitat. From a sedimentary perspective, the infiltration and accumulation of fine sediments can result in a reduction of the pore space and limit the transport of oxygen-rich surface water in the interstitials of riverbeds. From a biogeochemical perspective, the increased surface area for microbial growth can lead to an increase of respiration rates, which additionally limits the oxygen supply. The assessment and prediction of such processes on interstitial habitat quality represents a challenging task given their complex dynamic interacting processes and their high spatio-temporal variability. This study presents a new habitat-based modelling approach, which simulates interstitial habitat suitability (IHS) to evaluate dynamically the quality of interstitial habitat conditions during incubation. For this purpose, three key parameters (hydraulic conductivity, interstitial temperature and hyporheic respiration) are linked to the habitat requirements of different developmental stages during the incubation period (egg, hatching, larvae) via a multivariate fuzzy approach. The proposed modelling concept has been developed on the River Spoel in Switzerland, whereby results of a numerical 3D sediment transport model, together with supplementary measurements, deliver the spatio-temporal variations of the required input data. The fuzzy approach provides results in form of maps and time series of IHS values to allow for an identification of abiotic bottlenecks during the incubation period. Hence, this approach represents a significant contribution for the restoration of reproduction areas of gravel-spawning fish. Figure 2. Fuzzification of the three input parameters (hydraulic conductivity, interstitial temperature, hyporheic respiration) and of the output parameter (IHS). The graphs show overlapping membership functions, which are represented by linguistic terms (very low, low, medium, high and very high) M. NOACK ET AL.