Changing riverscapes and implications for ecological restorations from the analyses of freshwater mussels in the Kiso River, Japan

Large rivers have been modified for various human activities throughout the world, often resulting in a shortage of water and sediment, degradation of water quality and thereby modifications of habitat for aquatic biota. The Kiso River is the one of those whose water resources have been extensively utilized for a supply of water and electricity. Freshwater mussels are considered as organisms suitable to monitor the conditions of river ecosystems owing to their longevity, poor mobility, and co-evolutional relationships with fish. Despite the recognized trends of degrading riverscapes and quality of mussel habitats, there exists little quantitative information based on which ecological restorations should be planned. This study aimed 1) to quantify the changing riverscapes of a 25-km section of the Kiso River over the past 80 years, and 2) to relate the community structure of mussels to the historic and current status of respective habitat areas. Methods involved quantitative characterizations of the channel using a GIS with airphotos and results of routine surveys, and also field investigations on mussel communities and their habitat. Preliminary results over the past 25 years show that the riverbed level fell by 2 m and that gravel bars without vegetation cover decreased by >90%. The extent of vegetation encroachment to mussel-bearing areas appeared to have negatively correlated with the quality of mussel habitat. Therefore, any future restoration efforts such as artificial creations of mussel habitats may incorporate perspectives on the long-term and large-scale processes that have maintained gravel bars without extensive vegetation covers.