Restoration techniques for riverine aquatic connectivity: Current trends and future challenges in Japan

Abstract

Japan is known as a nation with high dam density. The mid- and downstream sections of most Japanese rivers are fringed by lateral embankments, causing serious deterioration of aquatic connectivity in the upstream-downstream, the longitudinal direction and the channel-floodplain, the lateral direction. The present review summarizes general causes of the fragmentation, restoration methods for riverine connectivity, and current status and future challenges of the restoration in inter-levee and outside-levee floodplains of Japanese rivers. In the inter-levee floodplain, we detected four restoration types pertaining to longitudinal fragmentation by physical and invisible barriers associated with water temperature and quality, and six restoration types concerning lateral connectivity. In marked contrast to the inter-levee floodplains, outside-levee floodplains have already been severed from a dynamic flooding process. In such altered systems, the management of an artificial habitat network such as a paddy-field or a pond is a major mechanism for creating alternative dispersal pathways. We also found that many of above restoration techniques present practical issues that limit the restoration effects on specific species or life stages and risk further degradation due to inappropriate selection of sites or procedures. Human pressure on freshwaters and the resilience of the ecosystems have been changing gradually, suggesting that Japanese river managers need to select appropriate techniques depending on these changes to restore the connectivity more efficiently. Realistically, "Habitat Creation" and "Partial restoration" focusing on the artificial or semi-natural connectivity are major strategies in the short run. On the other hand, "Full restoration" should be the major goal in a time span of a few decades, in the context of future social changes such as population decline and deterioration of infrastructures. In addition, a database of the location of low-head dams is needed to prioritize the restoration of the sites for connectivity both in the inter- and outside-levee floodplains.