Geographical patterns of flow-regime alteration by flood-control dams in Japan

Natural variability in water flow is an inherent feature of river ecosystems, but many rivers in the world have been dammed, altering the flow regime and leading to ecosystem degradation. Dam reoperation to maintain environmental flows has been proposed for ecosystem and biodiversity improvement. Reoperation requires an understanding of flow alteration due to dam operations. However, knowledge of natural flow regimes and their degree of alteration remain poor in Japan. The Japanese islands extend from the sub-Arctic to the subtropics with four seasons and thus are considered to show high spatial and temporal variation in flow regime and alterations caused by those variations. We examined the spatial and temporal dependency of flow alteration by flood-control dams in Japan. We confirmed that flood-control dams reduced the rate of change in discharge and peak discharge and that such dams fulfilled their primary role of preventing flood disaster by reducing and delaying flooding. However, other flow characteristics, such as high- and low-flow frequency and duration, were also altered. We demonstrated that the magnitude, frequency, duration, and seasonality of high and low flows in undammed flow regimes differed with latitude and that discharge alteration was spatially and temporally dependent, presumably because dam operation responded to the inherent differences in seasonal flow variability at different latitudes.