Improving restoration practice by deriving appropriate techniques from analysing the spatial organization of river networks

Abstract Amendments to the water protection legislation in many countries have raised the need to develop prioritization strategies in river restoration. These political objectives need to be translated into applied methods of site selection. The high degree of heterogeneity within administrative boundaries makes the identification of sites challenging. Analysing data with computer software alone might not identify sites with the highest ecological recovery potential, as they might not take sufficient account of the complex ecological interplay over large spatial scales. In this literature study, the spatial organization of river networks (dendritic structure, unidirectional flow, species distribution) is discussed in the context of different restoration techniques and how efficiency is expected to vary within the network. Although restoration planning must consider deficits on the reach scale, as well as catchment effects and develop suitable mitigation scenarios produced by the analysis, some general conclusions on the site-specific effectiveness of different restoration techniques can be derived from the spatial organization of river networks. Restorations in the headwaters are most suitable for improving fundamental ecological processes such as retaining nutrients and soils to improve water quality, buffering an increase of temperature by establishing riparian buffer-strips, and returning hydro-dynamic flow patterns to a more natural state by altered dam operation. Longitudinal connectivity is essential for many freshwater taxa and should be restored in a bottom-up direction, starting at the downstream ends of river networks or at species-rich nodes within the system. Habitat restorations and the re-establishment of a natural channel morphology throughout the network will aid ecological recovery, if species pools for re-colonization are close by and fundamental ecological processes support a recovery. To increase the success of future restoration efforts, branches of river networks should be seen as functional linked ecosystems, and therefore restoration efforts within one system should be more coordinated, rather than seeing every project as self-sufficient. There must be a shift from a tactical towards a strategic approach in river restorations.