Abstract
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 out flowing Seta River, restricting flow and increasing the tendency of small particles to be deposited on the floor of Lake Biwa. Inouchi 6 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.
Highlights
Sand beaches and sand-bottomed shallow sea areas form important habitat for fishes, shell fishes, shrimps, crabs and the larvae of some insects because these zones are rich in dissolved oxygen, which these species depend upon for respiration1. sand beaches are important for fish and biodiversity
An examination of sediment samples along the shoreline of Lake Biwa showed that the distribution of particles follows Stokes’ Law and forms a fan-shaped pattern centered on the river mouth
Based on samples taken before and after the construction of dams on the inflowing rivers, newly constructed dams were thought to carry greater quantities of particles smaller than MdΦ4, and the deposition of these smaller particles was governed by tidal current velocity with faster current carrying the particles farther from the shore
Summary
Sand beaches and sand-bottomed shallow sea areas form important habitat for fishes, shell fishes, shrimps, crabs and the larvae of some insects because these zones are rich in dissolved oxygen, which these species depend upon for respiration[1]. sand beaches are important for fish and biodiversity. Bays and harbors with vertical sea walls tend to have more bottom sediments and levels of dissolved oxygen that are lower than those found in these shallow sea areas with sand[1]. The average water level of Lake Biwa decreased about 50 cm after the Seta River was dredged in 1898 to 1906 and the old Araizeki Dam was constructed in 1905. The average water level of Lake Biwa decreased about 40 cm again after 1939 when the Seta River was dredged again[2]. It is thought that the present sand beaches of Lake Biwa are in the condition of those before 1898 because new particles are rarely deposited on beaches from rivers on which dams are constructed
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