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A field survey has been performed both in the estuary of the Banjo river and in the Saeki bay in order to investigate the effect of water quality on the clam habitat in tidal flat. The result of water quality measurements shows that dissolved oxygen depletes due to fine sediment in the bottom of the Banjo river. Dissolved oxygen profiles near the sediment/water interface on the tidal flat are simulated based on the experimental results of the oxygen consumption rate by the sediment. The dissolved oxygen concentration profile varies considerably near the sediment/water interface and rapidly becomes zero just below the interface regardless of the shear velocity, because all DO that reach the sediment surface are utilized instantly. There is not enough oxygen both at the sediment surface and inside the sediment. A tidal flat that has biologically active sediment accumulating is therefore an inappropriate place for the clam habitat.

and Kazuya MATSUMOTO Hydraulic structures such as a dam, weir and water gate affect on the flow depth, velocity, bed shape, water quality and so on, so that the hydraulic structures may affect on the water environment for plants and aquatic lives in an 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 it is not clear that what is the most suitable parameter which affects on the water environment. In this study, the most suitable parameter for spawning of Ayu were selected from the several values such as water temperature, velocity, flow depth, incline, BOD, COD, pH, SS and so on, on the basis of a principal component regression analysis. It was found that the most suitable parameters are temperature, velocity, flow depth and SS.