“DIF ESTATAL” declara que:

Hydroelectricity is a dominant source of renewable energy, which has the main advantage of quickly adapting to fluctuating market-dependent energy demand. During peak demand periods, high head storage hydropower plants (HPP) produce artificial discharge peaks, so-called hydropeaking, released into the river downstream. Though hydropower has a “green image” as it produces energy with low contributions to atmospheric CO2 emission, the unsteady water release from the tailwater outlet has severe cascading effects on the ecosystem downstream. In alpine regions like Switzerland, where hydropower production is economically highly relevant, every fourth river is affected by hydropeaking. This corresponds to approximately 1000 km of river reaches (Baumann & Klaus 2003). These unsteady water releases from reservoirs alter the diurnal and seasonal natural discharge regime of the rivers. A natural flow regime is a key factor for the ecological integrity of riverine ecosystem (Poff et al. 1997; Parasiewicz et al. 1998; Bunn & Arthington 2002). Hydropeaking cycles differ from natural floods because disturbance occurs sub-daily and at high amplitude (Limnex 2004). As a consequence, rivers with hydropeaking influence differ significantly in their species community compared to unimpaired rivers (Smokorowski et al. 2011; Young et al. 2011; Sanz 2012). To help in quantifying hydrological alteration of the natural flow regime caused by hydropeaking, hydraulic indicators have been developed. They describe sub-daily flow fluctuations in terms of intensity, frequency and the rate of flow changes (Meile et al. 2011). In addition to flow disturbance, hydropeaking also affects other abiotic conditions of the downstream sections. Water stored in high head reservoirs often has a different temperature than the receiving river, which results in daily intermittent temperature shifts, so-called thermopeaking and alters the seasonal temperature regime of downstream reaches (Zolezzi et al. 2011). Furthermore, particle transport is altered as a consequence of the sediment retaining capacity of reservoirs (Zwahlen 2003; Finger et al. 2006; Anselmetti

et al. 2007). Cascading effects affect other abiotic processes such as bed clogging, water

turbidity, stream bed particle size, velocity distribution, wetted area and hyporheic flow exchange (Gailiuis & Kriauciuniene 2009; Sawyer et al. 2009).

The hydrology, sediment regime, hydraulics and morphology determine the physical habitat available for stream organisms. In hydropeaking conditions, these parameters are strongly modified and exceed tolerance abilities of organisms, leading to drastic consequences on species diversity and abundance (Cushman 1985). Thus, the distribution of sensitive invertebrate taxa, macrophytes and fish are reduced (Moog 1993; Bernez & Ferreira 2007; Smokorowski et al. 2011). Healthy fish populations contribute to freshwater biodiversity and are essential for ecosystem functioning. They furnish valuable ecosystem services, firstly by organism consumption which regulates food web dynamics, sediment bioturbation on stream bottoms, or as gene, energy and nutrient reservoirs (Holmlund & Hammer 1999). Secondly, healthy fish populations are vital for generating resources for human society through food production and recreational fishing. Due to their body size and sensitivity to many stressors, fish are a suitable indicator to study anthropogenic stress on natural ecosystems (Harris 1995). Several studies of hydropeaking influences report a reduction in fish population size due to a loss of habitat availability and quality compared to natural rivers (Moog 1993; Smokorowski et al. 2011). As high head storage dams are situated in the mountainous regions, where steep and fast flowing headwaters dominate, some fish families are more strongly impacted.

Flow instability and brown trout reproduction success

Headwaters are typically inhabited by salmonid species, in Switzerland by brown trout (Salmo trutta). Fish movement is increased to adapt to sub-daily flow fluctuations (Scruton et al. 2003; Scruton et al. 2008). These extra movements lead to an increase in fish activity, which reduces energy reserves and impacts over-wintering survival of individuals. During winter, the spawning period for salmonids, natural discharge is usually low. Therefore differences between peak and off-peak discharge is high. Under these conditions, spawning behavior is altered (Chapman et al. 1986), redds and egg pockets are exposed to dewatering which may result in the death of the eggs (McMichael

et al. 2005). It is known that fine sediment accumulation reduces oxygen supply to the

embryo either by bed clogging or reduction of egg oxygen exchange through the membrane and thus affects embryo survival (Greig et al. 2005a; Jensen et al. 2009; Yamada & Nakamura 2009). However, egg survival under altered particle transport, resulting from HPP operation, is still poorly understood. From emergence to the end of their first winter, young-of-the-year (YOY) stay in shallow riverbank habitat, where flow velocity is low (Crisp 2000). In rivers influenced by hydropeaking, the growth rate, density and mesohabitat choice of juveniles is affected (Jensen & Johnsen 1999; Flodmark et al. 2006; Korman & Campana 2009). In addition, a physiological stress response was observed to fish after exposure to hydropeaking. However, Flodmark et al. (2002) showed that juvenile can adapt to such conditions and stress response decreases with increased exposure time. Shallow and irregular riverbanks combined with high discharge peaks lead to extra movements as well as drift and the risk of stranding of young fish (Liebig et al. 1998; Halleraker et al. 2003). However, stranding and drift risks strongly depend on riverbank slope, substrate type, shelter availability as well as amplitude, magnitude, duration, frequency and speed of up and down ramping (Liebig et

al. 1998; Saltveit et al. 2001; Halleraker et al. 2003; Berland et al. 2004).

In the past years, habitat modeling has become an important tool for evaluating the impact of human-altered flow regime to the fish fauna. Such modeling studies reported unsteadiness in adult fish habitat under hydropeaking conditions (Valentin et al. 1996; Person & Peter 2012) (see chapter 3). Instream models are based on: 1) hydrodynamic model: simulating spatial and temporal variations in abiotic parameters (depth, velocity and substrate conditions), 2) abiotic preferences for the target fish species, 3) physical habitat model, combining the results of the hydrodynamic model and the biological preference (Bovee et al. 1988). Habitat suitability for the target fish species is modeled for varying flow conditions. CASiMiR is a habitat simulation system from the instream model family, including a fish module especially developed to model habitat suitability at different flow rates (Jorde et al. 2000; Schneider et al. 2001). This module was applied to study habitat evolution of 8 Chilean endemic fish species under hydropeaking regime and suggest appropriated habitat improvement measures (Garcia et al. 2010). These studies were mainly focused on adult fish and therefore lack in considering other life stages (Valentin et al. 1994; Valentin et al. 1996).

Weighted Usable Area (WUA) and Hydraulic Habitat Suitability (HHS) are used to quantify the amount of suitable habitat in PHABSIM habitat modeling approaches.

Chapter 4

calculated based on the abiotic preferences of those species. This preference is described by habitat suitability curves (HSCs). The product of habitat use by the organisms over habitat availability, in the ecosystem is calculated. Habitat preferences of the same fish species differ between regions due to regional adaptation and plasticity. Therefore, habitat modeling results rely strongly on the chosen HSCs (Heggenes et al. 1996). As HSCs are commonly not available for the studied river or are not known, most studies rely on expert based knowledge or published HSCs of other catchments, which are subsequently adapted for a specific geographic area (Valentin et al. 1996; Ovidio et al. 2008).

In the “Green Hydropower” assessment procedure for river management, hydropeaking was identified as one of the future research priorities. This is because of the lack of knowledge concerning its interactions with the river ecosystems downstream and thus the difficulty to identify appropriate mitigation approaches (Bratrich et al. 2004). The importance of mitigating human impacts on river ecosystems has been recognized and resulted in the initiation of water protection policies such as the Water Framework Directive of the European Union and the new water protection law in Switzerland (LEaux (OFEV 2009)). In the Swiss water protection law, hydropeaking is recognized to cause serious infringement on the downstream river. A harmful threshold of 1:1.5 was defined for the off-peak:peak ratio. Different mitigation measures are proposed in a strategic plan to reduce negative effects resulting from hydropeaking (Sanierung Schwall/Sunk – Strategische Planung, 2012). This involves operational measures (change in turbine operation) as well as structures that buffer flow peaks, such as compensation basins or multipurpose schemes (Heller & Schleiss 2011). Both strategies should reduce the hydrological impact of HPP operation. Morphological measures, such as river revitalisation are also considered, as far as they mitigate hydropeaking effects by increasing natural retention capacities of rivers (Church 1995). Such morphological habitat enhancement measures performed on the Oulujoki River in Finland contributed to the maintenance of a grayling population under HPP operation (Vehanen et al. 2003). However, Weber et al. (2007) argued that morphological improvements are not sufficient for a successful rehabilitation if the hydrological regime remains altered. There is a clear need for quantitative framework studies, incorporating simulation and modeling approaches as well as biological monitoring methods.

This chapter focus on the hydropeaking effects on the early and sensitive life stages of brown trout (spawning and young-of-the-year). The effect of sub-daily flow fluctuation on the natural reproduction is studied in a morphologically natural river. The role of natural morphology as hydropeaking mitigator by directly influencing velocity, depth and grain size distribution in the river bed and consequently sustain fish spawning and nursery habitat is investigated. A theoretical habitat model approach was combined with observation and field experiments. Habitat suitability and stability is modeled with the CASiMiR fish module. For this purpose, specific HSCs have been developed for the investigated river for spawning and YOY life stages. Habitat changes are quantified with dynamic habitat descriptors developed by Person et al. (2013) (see chapter 6) assessing habitat loss (WHL) and habitat dewatering (DAR) especially developed for modeling fluctuating flow conditions. Natural reproduction success is investigated with in situ egg to hatching incubation experiments and YOY density sampling surveys.

Flow instability and brown trout reproduction success