Current World Environment

The development of sand littoral zones is critical to supporting specific species in lakes and oceans. The construction of dams on rivers changes the distribution of sediments in littoral zones, and the relationship between dam construction on rivers, the inflow of small particles and increased eutrophication and red tide occurrences was demonstrated for Lake Biwa using public data. Many dams were constructed on rivers around Lake Biwa after the Second World War, and the old and new Araizeki dams were constructed on the outflowing Seta River, restricting flow and increasing the tendency of small particles to be deposited on the floor of Lake Biwa. Inouchi (J. of Geol. Society of Japan, Vol. 88, No. 8, 1982, pp. 665-681) reported the distribution of seafloor sediment particle sizes in the Seto Inland Sea. Inouchi showed several fan-shaped distributions of sediment particles centered at the mouths of rivers. After many dams were constructed on the rivers in the period following the Second World War, particles smaller than MdΦ 4 to 6 were thought to increase in the rivers, and these smaller particles were deposited farther offshore from the river mouth if tidal currents were faster than 0.5 to 1.0 knots. Areas of the Seto Inland Sea in 1975 that were affected by silting and subsequent red tide blooms include Hiroshima Bay, Hiuti-nada, Harima-nada and Osaka Bay. These findings and similar patterns between the Seto Inland Sea and Lake Biwa support my hypothesis that the influx of mud due to the construction of dams brings about eutrophication and red tides.