Return period of flood disturbance that increases diversity on gravel bars in middle stream of rivers

Although medium-class flood disturbance is highly correlated with diversity of vegetated area of river habitat, it is still not clearly defined due to its complexity. To describe flood disturbance characteristics, two indices, breakage or overturning index and washout index, are used to express the breaking condition of trees and washout condition of trees and grasses. The relationship between diversity index of vegetated area (DI), calculated using vegetation species maps, and flood disturbance index (I _i ), which represents the expected value of different flood disturbances, was investigated on four gravel bars in the Karasugawa River, Japan. The flood disturbance indices in Region A (high possibility to be a forest) and Region E (high possibility to be a bare area, as defined in this study) are identified as indicators for medium-class flood disturbance in this study. Moreover, this study confirms the results of previous studies on the Arakawa and Tamagawa Rivers, Japan, and their applicability to other rivers. In addition, the relationship between flood disturbance and DI in Region A or E explains the effect of flow regulation by construction of dams and weirs in river upstream on the downstream river habitat. Furthermore, based on the observed data and calculated results related to the possibility of vegetation and bare area on gravel bars in all investigated rivers, a range of flood return periods for medium-class flood disturbance for habitats on gravel bars was defined as 3–12.5 years, where the lower and upper values are related to the possibility of the presence of vegetation and the possibility of being a bare area, respectively.