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Abstract: Hydraulic structures such as a dam, weir and water gate affect on the flow depth, velocity, bed topology, water quality and so on, so that the hydraulic structures may affect on the water environment for plants and aquatic lives in and around rivers. Therefore, before construction of such hydraulic structures, it is necessary to conduct the environmental assessment. However, the accuracy of the environmental assessment is not so high at present, because the preference curves of the flow depth, velocity and so on are not clear. Recently, Nagaya et al. [Nagaya, T., Onitsuka, K., Higashino, M., Takami, T., Otsuka, N., Akiyama, J., Matsumoto, K., 2004. Evaluation of weight of parameter for spawning of ayu, Plecoglossus altivelis, and prediction of spawn density per unit area. In: Lee, J.H.W., Lam, K.M. (Eds.), Environmental Hydraulics and Sustainable Water Management, vol. 2. AA. Balkema Publishers, UK, pp. 947–954] pointed out that the velocity, flow depth and substrate affect on the suitability of spawning for ayu, Plecoglosses altivelis altivelis on the basis of statistical investigation. Preference curves of velocity and flow depth were made clear by Onitsuka et al. [Onitsuka, K., Nagaya, T., Higashino, M., Takami, T., Otsuka, N., Akiyama, J., Ozeki, H., Matsumoto, K., Shiraishi, Y., 2005. Suitable flow depth and velocity for spawning of ayu, Plecoglossus altivelis. In: Proceedings of the 31th Congress of IAHR, Korea, pp. 1850–1858]. On the other hand, the preference curve of substrate has not been investigated. In this study, the preference curve of substrate was investigated on the basis of field survey. Further, a numerical simulation by making use of a horizontal 2D numerical model was carried out with changing the discharge at the Gokasegawa River due to a lot of ayu inhabits in the Gokasegawa River. The numerical model is based on the model which was suggested by Nagata et al. [Nagata, N., Hosoda, T., Muramoto, Y., 2000. Numerical analysis of river channel processes with bank erosion. Journal of Hydraulic Engineering, ASCE 126 (4), 243–252]. The suitability of spawning for ayu is predicted with the preference curves of the flow depth, velocity and substrate. [Copyright &y& Elsevier]