Abstract. Global warming has already affected European rivers and their aquatic biota, and climate models predict an increase of temperature in central Europe over all seasons. We simulated the influence of expected changes in heat wave intensity during the 21st century on water temperatures of a heavily impacted pre-alpine Austrian river and analysed future mitigating effects of riparian vegetation shade on radiant and turbulent energy fluxes using the deterministic Heat Source model. Modelled stream water temperature increased less than 1.5 • C within the first half of the century. Until 2100, a more significant increase of around 3 • C in minimum, maximum and mean stream temperatures was predicted for a 20-year return period heat event. The result showed clearly that in a highly altered river system riparian vegetation was not able to fully mitigate the predicted temperature rise caused by climate change but would be able to reduce water temperature by 1 to 2 • C. The removal of riparian vegetation amplified stream temperature increases. Maximum stream temperatures could increase by more than 4 • C even in annual heat events. Such a dramatic water temperature shift of some degrees, especially in summer, would indicate a total shift of aquatic biodiversity. The results demonstrate that effective river restoration and mitigation require re-establishing riparian vegetation and emphasize the importance of land-water interfaces and their ecological functioning in aquatic environments.

-Stream water temperature limits the growth and survival of aquatic organisms; whereby riparian shading plays a key role in inhibiting river warming. This study explains the effects of riparian shading on summer water temperatures at a pre-alpine Austrian river, during heatwave and nonheatwave periods at low flow conditions. A vegetation-shading index was introduced for the quantification of riparian vegetation effects on water temperature. For maximum water temperatures, a downstream warming of 3.9°C was observed in unshaded areas, followed by a downstream cooling of 3.5°C in shaded reaches. Water temperature directly responded to air temperature and cloudiness. For an air temperature change of 2°C we modelled a water temperature change of 1.3°C for unshaded reaches, but lower changes for intensively shaded reaches. Similar daily variations at shaded reaches were up to 4°C lower than unshaded ones. This study gives clear evidence that for a medium-sized prealpine river, restoration practices should consider that discontinuity of riparian vegetation should be less than 6000 m; with more than 40% dense vegetation in order to minimize water temperature increases due to unshaded conditions. Keywords: riparian vegetation / vegetation-shading index / water temperature / river vegetation management / restorationRésumé -L'influence de la végétation rivulaire sur la température de l'eau pendant les conditions de faible débit dans une rivière de taille moyenne. La température de l'eau d'une rivière limite la croissance et la survie des organismes aquatiques ; de ce fait l'ombrage rivulaire joue un rôle clé dans la limitation du réchauffement des rivières. Cette étude analyse les effets de l'ombrage rivulaire sur les températures estivales de l'eau dans une rivière autrichienne préalpine, pendant les périodes de canicule et de non-canicule à faible débit. Un indice d'ombrage de la végétation a été introduit pour la quantification des effets de la végétation rivulaire sur la température de l'eau. Pour les températures maximales de l'eau, un réchauffement en aval de 3,9°C a été observé dans les zones non ombragées, suivi d'un refroidissement en aval de 3,5°C dans les zones ombragées. La température de l'eau a répondu directement à la température de l'air et à la nébulosité. Pour un changement de température de l'air de 2°C, nous avons modélisé un changement de température de l'eau de 1,3°C pour les zones non ombragées, mais des changements plus faibles pour les niveaux intensivement ombragés. Des variations quotidiennes semblables dans des zones ombragées étaient jusqu'à 4°C inférieures à celles non ombragées. Cette étude montre clairement que pour une rivière préalpine de taille moyenne, les pratiques de restauration devraient considérer que la discontinuité de la végétation riveraine doit être

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Abstract. The influence of expected changes in heat wave intensity during the 21st century on the temperatures of an pre-alpine river are simulated and the mitigating effects of riparian vegetation shade on the radiant and turbulent energy fluxes analysed. Minor stream water temperature increases are modelled within the first half of the century, but a more significant increase is predicted for the period 2071–2100. The magnitude of maximum, mean and minimum stream temperature rises for a 20 year return period heat event was estimated to be in the region of 3 °C. Additional riparian vegetation is not able to fully mitigate the expected temperature rise caused by climate change, but can reduce maximum, mean and minimum stream temperatures by 1 to 2° C. Removal of existing vegetation amplifies stream temperature increases. Maximum stream temperatures could increase by more than 4 °C even in yearly heat events.