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History of Dam Construction and Water Quality Issues - Summary In 1999, severe blue-green algae blooms caused by eutrophication occurred at Shichikashuku Dam in Miyagi Prefecture, with phosphorus concentrations exceeding the standard value by more than 20 times. At Tokuyama Dam in Gifu Prefecture, sediment inflow caused agricultural damage exceeding 100 million yen annually. In the Kiso River basin of Nagano Prefecture, declining groundwater levels impacted drinking water supply. In the 2000s, countermeasures such as circulation systems and sediment removal progressed, but their effects were limited. The 2010s saw the experimental introduction of phosphorus adsorbents and groundwater recharge models, leading to improvements in some areas. However, environmental impacts and effects on local communities remain ongoing challenges. Continuous environmental protection and sustainable water resource management are required.