Author(s): Shigeya Nagayama; Morihiro Harada; Shiro Sagawa; Yuichi Kayaba
Linked Author(s): Morihiro Harada
Keywords: No Keywords
Abstract: The riverscape in Japan has changed significantly since the Meiji era (1868–1912) when modern civil engineering technology was introduced from Europe. The floodplain, which was a broad wetland on the coastal alluvial plain, has been disconnected from the main river channel due to intensive levee construction from the Meiji to the Showa (1926–1989) periods. Since the 1980s, inter-levee floodplains in many Japanese lowland rivers have been terrestrialized, with riparian forests established in the context of riverbed degradation. The inter-levee floodplain is artificial but is now recognized as a rare habitat for a diverse range of aquatic organisms, including endangered species such as freshwa ter mussels and bitterlings. However, terrestrialization and riparian forest establishment have led to a deterioration of inter-levee floodplain ecosystems. In addition, they have decreased flow discharge capacity in inter-levee zone. River managers have often implemented tree removal and excavation of terrestrialized floodplains (i. e., flood channel) to increase flow discharge capacity. Flood channel excavation with tree removal is currently the primary and most practical method for flood control in Japan and it may support recovery of floodplain ecosystems due to increases in inundation area and frequency. In this study, this hypothesis was examined through the recovery of freshwater unionid mussels, which are potential indicator species of floodplain ecosystems. To further the understanding of inter -levee floodplain conservation through flood channel excavation, we examined the relationships between mussel habitat (floodplain ponds naturally created post excavation) and the initial ground elevation and elapsed time (max. 12 years) after excavation. This study was conducted in the lowland Ibi River (32–39 km from the river mouth) in central Japan. The riverbed slope was approximately 1/3,300. The excavation work was implemented in the study area between 2000 and 2007 to increase flow discharge capacity. Eleven excavation sites were selected, with different initial ground elevations and different time periods elapsed since excavation. Drought, low, ordinary, and high water levels were used as a measure of the initial excavation height. The river managers did not expect the recovery of floodplain-dependent aquatic organisms. However, freshwater mussels eventually colonized ponds, which were naturally created in the excavation sites through flooding. Mussel surveys of 80 ponds were conducted in nine of the excavation sites in December 2011. In all 11 excavation sites, cumulative sediment depth and floodplain pond area were investigated based on cross-sectional profiles (2001,2002,2005,2008, and 2013) and aerial photographs (2002,2006,2007,2009, and 2012). Two generalized linear mixed models (GLMM) were built, in which the response variables were cumulative sediment depth (m) and pond area (m2); a generalized linear model (GLM) was also built, in which the response variable was mussel abundance (CPUE: catch per unit effort, N/hour). The explanatory variables used were initial excavation height and time elapsed (years) after excavation.
Year: 2018