Observations of Physical Environments of Groin Areas in Tidal Regions of the Asahi River

Abstract

Water regions where groins are constructed in tidal rivers are very important habitats for fishes and birds. The purpose of this paper is to investigate the physical environments of such regions in the tidal region of the Asahi River.
A stable halocline is usually formed in the regions between the groins. Tidal processes move the highly saline water into and out of the lower layer in this region. The groins appear above the water surface during the ebb stage, and disappear during the flood stage. When the groins are submerged, water with low salinity from the upper layer rapidly flows over the top of the groins in the same direction as the main river current. During ebb stage, most of the highly saline water in the lower layer flows out of the groin area. The retention time of water in the region between the groins is relatively short owing to the rapid water exchange caused by tidal processes. However, dissolved oxygen concentration often decreases in the lower layer. This decrease appears to be the result of oxygen deficient water flowing horizontally along the shallow bottom from the main river, where an oxygen deficient water mass caused by chemical and biological pollution appears.