The Watershed Fragmentation by Dams and Its Impacts on Freshwater Fishes

Interactions between geomorphology, land cover, food webs, and material flows are important when designing an integrated watershed management plan. River networks, which are the fundamental structure controlling these interactions, have been fragmented by a large number of dams and other artificial barriers constructed throughout Japan, isolating terrestrial habitats from marine habitats.

The objective of this study is to assess quantitatively the degree to which rivers and watersheds are fragmented by damming and the effect on freshwater fish distribution. A new GIS methodology was developed to delineate fragmented watersheds and to determine the periods of fragmentation for each watershed based on location and the construction year of a dam. This methodology was applied to entire river networks and large dams (more than 15 meters high) nationwide to visually understand fish habitat degradation associated with damming. Detailed examination of the damming effect on fish distribution was conducted exclusively for the rivers in Hokkaido where a database of existing fish sampling data is available.