Restoration of the fish and invertebrate habitat: The re-meandering project of a large river in northern Japan

Background/Question/Methods Since the 1960’s, the Shibetsu River in northern Japan has been channelized and the floodplains significantly altered to accommodate cattle grazing and crop cultivation, resulting in the loss of riverscape heterogeneity and the subsequent loss of aquatic and terrestrial habitats due to severe damage of the river ecosystem. Stream channelization for flood control diminishes the inundation frequency of a floodplain by isolating the aquatic habitats distributed across it or by reducing the connectivity among them. Inundation frequency reduction also leads to the disruption of the geomorphic processes that create floodplain habitats, thereby eliminating lateral habitat variation. In 2008, the Shibetsu River Restoration Project initiated the restoration of the meander course by connecting a 3.5 km channelized reach with a series of oxbow lakes. Prior to full-scale restoration, we implemented an experiment at a channelized reach by connecting an oxbow lake (previous meander reach) in the spring of 2002. Several studies have reported the effects of meander restoration on fish and macro-invertebrate assemblages in lowland rivers but none have analyzed changes in hydraulic parameters associated with re-meandering. Thus, relationships between changes in the physical environment and stream biota remain ambiguous We first examined how riverscape structure and function have changed with land use development and river channelization over the last 50 years, to understand the ecological roles of meandering rivers. We next examined how fish and macro-invertebrate assemblages responded to changes in the physical environment associated with the re-meandering project.

Results/Conclusions Results clearly indicated that recovery of geomorphic dynamics could play significant roles in the restoration of river ecosystems. The riverscape of a natural meandering reach, previously observed everywhere in the downstream section of the Shibetsu River, contains various landscape units such as bank edge, point bar edge, glide, backwater, side channel, oxbow, and these units including woods. Side channels and units with woods were used by abundant fish species. Oxbows provided habitats suitable for lentic fish. Each of the other units functioned as a habitat for different fish assemblages according to their hydrogeomorphic condition. The channelized reach consisted of only several simple units, however, and few fish species could exist there. The re-meandered reaches develop shallow-edge habitats with low hydraulic stress along the inside of a convex, providing stable substrates for macro-invertebrate communities. Fallen trees provided by lateral erosion, promoted macro-invertebrate colonization and became pool habitats and in-stream cover for fish species at different life stages.