Quantifying impacts of dams and dam removal on riverine systems

Dams are one of the biggest threats to biodiversity and functioning of riverine ecosystems. Although the construction of new dams continues, awareness of environmental impacts has resulted in an increasing number of dams being decommissioned and removed. Quantification of local to ecosystem-level (e.g. catchment-based) impacts is needed to validate key assumptions in common mitigation and restoration schemes. Understanding biological responses to dams and dam removal are increasingly relevant for quantifying local- to ecosystem-level impacts because:

1. the spatial scale of responses to dams and dam removal correspond to the spatial scale of impact,
2. dam removals provide benchmarks of the potential for environmental flows to mitigate impacts of dams on biodiversity and function, 
3. dam removals reduce cumulative effects of multiple barriers in stream networks.

Schematic illustration of both longitudinal, and lateral impact of dams on ecosystems.

Despite years of research on dams and dam removal critical knowledge gaps remain hindering application of regulations and standards. In this project, we will make general predictions of biological responses to dams and dam removal in a quantitative review (metaanalyses) with fish and macroinvertebrates as focal organism groups. The composition of fish and macroinvertebrate communities, and their assemblage-derived metrics, reflects the local- to ecosystem-level physical and chemical conditions. This makes them highly sensitive to detection of the impairment and improvement of aquatic systems, and ideal organisms for indicating how such highly dynamic spatiotemporal changes brought about by dams and dam removal will affect the ecosystem and the potential of dam removal as a restoration and mitigation measure.

Organisms that will be included in the study on dams and dam removal. Collage with photos by: SLU, and HHelene, Manatus and X-pixell on Shutterstock.

Statistical models will include a space–time component and explanatory factors that describe dam characteristics and environmental settings. Our study will identify systemic consistencies in biological responses and key environmental drivers between the studies. Further, we focus on areas of research urgently needed by decision makers and other stakeholders.

Our research focus is to validate key assumptions in common mitigation and restoration schemes, such as:

1. the rate and spatial extent (longitudinal and lateral) of instream and stream-riparian meta-ecosystem responses to dam removal, and when to anticipate negative effects,
2. the potential for environmental flows to improve biodiversity and function in hydromorphologically (physical character and water content) degraded systems,
3. the identification of biological metrics and indices that are suitable to characterize and quantify hydromorphological degradation as well as improvement.

One of the key outcomes of this project will be guidance on mitigating dam impacts. The guidance can be used in water management on different levels.