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s u m m a r y Impacts of anthropogenic flow regulation on the thermal regimes of alpine river systems are poorly understood. This is surprising given the importance of water temperature for river ecosystems and the widespread regulation of mountain rivers across the world. This study examined water temperature dynamics year-round between July 2008 and September 2009 in the Eisboden river system, central Austrian Alps. Water temperature data were examined alongside hydroclimatological data to infer the key processes driving thermal variability from diurnal to inter-annual scales. As expected, interactions between meteorology and water source controlled year-round thermal heterogeneity. However, water entering the proglacial river from a hydropower storage reservoir caused significant increases in water temperature during both late summer and early winter, resulting in a marked longitudinal thermal discontinuity. The timing and duration of flows discharged from reservoirs, and thus effects on river thermal regimes, differed considerably from previous studies of subalpine hydropeaking. Furthermore, thermal responses to flow regulation extended laterally to some groundwater tributaries even where there was no upstream surface connectivity, suggesting significant hyporheic flow or conduction of heat through coarse alluvium. River water temperature continued to be altered even after reservoir releases had ceased due to the removal of winter snow cover and recharged groundwater sources. Together, these insights into the thermal variability have broad implications for conservation and management of alpine river systems because water temperature is a key variable influencing aquatic ecosystems, and because anthropogenic pressures on alpine environments are expected to grow in the future.

Alpine glacier-fed river hydrology, chemistry, and biology can vary significantly in 17 space and over diurnal to interannual time scales as a function of dynamic inputs of water from 18 snow, ice, and ground water. The sensitivity of biota to these water-source dynamics potentially 19 makes them susceptible to hydrological changes induced by anthropogenic activities, such as 20 flow regulation, but most alpine studies have been focused on intact rivers during summer only. 21We examined the spatiotemporal dynamics of physicochemical habitat and macroinvertebrate 22 assemblages in a high-altitude (>2000 m) floodplain in the European Alps over an 18-mo period. 23We present a novel insight into the river system and macroinvertebrate assemblage responses to 24 natural glacier-melt-driven expansion-contraction of unregulated river sites and to intermittent unaffected by reservoir releases despite significant increases in water temperature and discharge 35 at these times. The effects of alpine river management for hydropower production on 36 macroinvertebrate assemblages in this river system appear to be relatively minor, but further 37 studies should be undertaken in other alpine locations to assess the generality of this finding. 38Key words: flood-pulse, glacier, groundwater, hydropower, macroinvertebrate, reservoir 39 FWS MS 14-043 3 Alpine zones are found on all continents between the treeline and permanent snowline, 40 and they host an array of glacier-melt, snowmelt, groundwater, and mixed-source rivers that 41 provide considerable heterogeneity of habitat and biodiversity (Füreder 1999, Brown et al. 42 2003). Recent research has highlighted the potential loss of biodiversity from these systems with 43 glacier retreat via alterations to river flow, water temperature, geomorphology, and water 44 chemistry (Brown et al. 2007, Jacobsen et al. 2012, Cauvy-Fraunié et al. 2014. Alpine aquatic 45 ecosystems appear to be particularly sensitive to environmental change because of strong system 46 linkages between climate, water sources, physicochemical habitat conditions, and biodiversity 47 (Hannah et al. 2007, Brown et al. 2009). Understanding of these linkages has developed mostly 48 from space-for-time approaches, often used along gradients of meltwater contribution, catchment 49 glacial cover, or multivariate glaciality indices (Milner et al. 2009). However, to date, detailed 50 gradient studies on alpine rivers have focused on data collected during the summer melt season 51 only. Thus, such approaches need to be evaluated more thoroughly over annual time scales. 52Glacial river systems exhibit considerable seasonal physicochemical habitat change 53 associated with the glacial flood pulse (Malard et al. 2006, Cauvy-Fraunié et al. 2014 (Fig. 1). 114We monitored 6 sites along part of the Eisboden River, which is sourced from the on when valves were opened in winter by the hydropower company (Fig. S1). 126River environmental variables and benthic macroinvertebrate were sampled on 8 (Table S1). Sites...

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Estuaries are potentially exposed to compound flooding where weather-driven extreme sea levels can occur synchronously with extreme fluvial discharge to amplify the hazard. The likelihood of compound flooding is difficult to determine due to multiple interacting physical processes operating at sub-daily scales, and poor observation records within estuaries with which to determine potential future probabilistic scenarios. We hypothesize that fluvial extremes can occur within the peak of the surge in small/steep catchments because of rapid runoff times, whilst the length-scale in larger/flatter catchments will result in fluvial and marine extremes being out-of-phase. Data (15 min river flow and hourly sea level) spanning 40 years were analyzed to assesses the behaviour and timings of fluvial and sea level extremes in two contrasting estuaries: Humber and Dyfi (United Kingdom). Compound events were common in the Dyfi, a small/steep catchment on Britain’s west coast with fast fluvial response times. Almost half of the 937 skew-surge events (95th-percentile) occurred within a few hours of an extreme fluvial peak, suggesting that flood risk is sensitive to the storm timing relative to high tide—especially since flows persisted above the 95th-percentile typically for less than 12 h. Compound events were more frequent during autumn/winter than spring/summer. For the Humber, a larger/flatter catchment on the east coast with slower fluvial response times, extreme fluvial and skew-surge peaks were less frequent (half as many as the Dyfi) and compound events were less common (15% of events co-occurred). Although flows in the Humber persisted above the 95th-percentile for typically between one and 4 days, hence overlapping several high tides and possibly other surges. Analysis of 56 flooding events across both estuaries revealed: 1) flooding is more common in the Dyfi than Humber; 2) Dyfi flooding is driven by 99th-percentile flows lasting hours and co-occurring with a 95th percentile skew-surge; 3) Humber flooding was driven by 95th-percentile flows lasting days, or surge-driven—but rarely co-occurring. Our results suggest that compound flooding studies require at least hourly data (previous analyses have often used daily-means), especially for smaller systems and considering the potential intensification of rainfall patterns into the future.