"The water runs the river." This chapter focuses on the river flow as the fundamental process determining the size, shape, structure, and dynamics of riverine ecosystems. We briefly introduce hydrological regimes as key characteristics of river flow. Hydrological regimes are then linked to habitats and biotic communities. The effects of flow regulation as a result of human activities such as water abstraction (irrigation and hydropower), river channelization, land use, and climate change are demonstrated. Finally, methods to assess the environmental flow, the flow that is needed to maintain the ecological integrity, are described, and examples of successful flow restoration presented. The Water Cycle and Hydrological RegimesIn temperate zones water received via precipitation is either stored in ice and snow during winter or infiltrates into the groundwater and is released into rivers during summer. Water cycles through stages of evaporation, water storage in the atmosphere, precipitation, (sub)surface runoff, and storage in the ocean. The water cycle and climatic conditions form the boundary conditions for the hydrological regimes that define distinct seasonal and daily flow patterns. High altitude rivers receive water mainly from glacial melt during summer with distinct diurnal melting peaks following air temperature warm-up (glacial regime) (Fig. 4.1). At lower elevations snow melting in spring causes seasonal peaks (nival regime), while periods of high flow and floods due to rainfall can occur at any time of the year (pluvial regime).

Abstraction, diversion, and storage of flow alter rivers worldwide. In this context, minimum flow regulations are applied to mitigate adverse impacts and to protect affected river reaches from environmental deterioration. Mostly, however, only selected instream criteria are considered, neglecting the floodplain as an indispensable part of the fluvial ecosystem. Based on essential functions and processes of unimpaired temperate floodplain rivers, we identify fundamental principles to which we must adhere to determine truly ecologically-relevant environmental flows. Literature reveals that the natural flow regime and its seasonal components are primary drivers for functions and processes of abiotic and biotic elements such as morphology, water quality, floodplain, groundwater, riparian vegetation, fish, macroinvertebrates, and amphibians, thus preserving the integrity of floodplain river ecosystems. Based on the relationship between key flow regime elements and associated environmental components within as well as adjacent to the river, we formulate a process-oriented functional floodplain flow (ff-flow) approach which offers a holistic conceptual framework for environmental flow assessment in temperate floodplain river systems. The ff-flow approach underlines the importance of emulating the natural flow regime with its seasonal variability, flow magnitude, frequency, event duration, and rise and fall of the hydrograph. We conclude that the ecological principles presented in the ff-flow approach ensure the protection of floodplain rivers impacted by flow regulation by establishing ecologically relevant environmental flows and guiding flow restoration measures.

Hydropower (HP) is an important renewable energy source contributing 65.7% to Austria's national electricity generation. However, HP is also associated with ecosystem degradations jeopardizing the aims of the EU Water Framework Directive (WFD) and Habitats Directive.Based on the EU Renewable Energy Directive (RED), the Austrian Energy Strategy has defined goals to further increase HP production by 3.5 TWh until 2015. Because national strategies for HP development are widely missing, hydropower plants (HPPs) are planned and approved on a local and regional level, often neglecting the overall optimum for energy supply and ecology. Therefore, a decision support tool (Hy:Con) was developed to integrate the energy-economic characteristics of planned HPPs and conservation needs of ecologically sensible river stretches. Based on 102 planned HPPs in Austria, Hy:Con identified HPPs with high economic attractiveness and low conservation concerns. The results show that owing to the already high HP exploitation in Austria, only a minor number of projects are without conservation conflicts. Upgrading of existing HPPs was associated with least ecological impacts, while HPPs with reservoirs are favoured over run-of-river plants. Cumulated ecological effects of numerous small HPPs are significant, whereas their contribution to overall energy production is comparatively small. Hy:Con represents a strategic instrument that can help decision makers to govern the implementation of the RED and WFD in a transparent way to pinpoint the limitations of future HP development and to avoid conflicts and stranded investments.

Rivers have all too often been considered as two-dimensional elements of terrestrial landscapes neglecting their own internal structure and heterogeneity. But rivers exhibit certain characteristics, which should grant them a special position in connectivity conservation. With habitat fragmentation causing dramatic losses in global aquatic biodiversity, ecological research put much effort into conservation measures for maintaining and restoring connectivity of riverine habitats. To make use of the full mitigation hierarchy, the implementation of both avoidance (e.g. large-scale planning) and mitigation measures (e.g. facilities for up- and downstream migration) should be aspired.

Rivers worldwide have been transformed into fragmented, impounded, channelized, and flow-regulated ecosystems. These anthropogenic transformations can reduce fish distribution and population status, especially of those species belonging to medium- or long-distance migratory guilds and those dependent on free-flowing rivers and intact sediment and habitat conditions. Here, we aim to understand how different hydro-morphological pressure types affect the distribution and population status of key potamodromous fish species of the rheophilic and lithophilic fish guilds, the barbel (Barbus barbus) and the nase (Chondrostoma nasus). We also assess the status of chub (Squalius cephalus) to include a species less sensitive to habitat degradation. For the first time, we assembled an extensive Austrian-wide GIS-based fish sampling database with hundreds of biological surveys, allowing us to analyze quantitatively >4,000 river kilometers for presence/absence of target fishes and to assess population status. The data reveal that the distribution range of target species decreased by around 40–60% compared to their natural ranges according to the reference standard (Leitbild). Hydro-morphological pressures affect target species’ population biomass, and trends between impact types can be detected. Chub and barbel exhibit the highest median biomass in free-flowing rivers and residual flow reaches, followed by reservoir sections. Of all pressure types, population biomass is lowest in hydropeaked river stretches. Nase biomass has a grand median of 0.0 kg/ha across all sites, showing hardly any differences between hydro-morphological pressure types. Overall, our results show a drastic shrinkage of the distribution range of three cyprinid fish species previously prominent in Austria. By linking current population vitality to hydro-morphological stressors and ecological status assessments, this study sets a baseline for data-based conservation actions of (Red-listed) species as well as policy and management frameworks.