page_title: Shihmen Dam
section_title: Water quality
hierarchical_section_title: Shihmen Dam / Environmental issues / Water quality
caption_attribution_description: English: Original description: 石門水庫 (Shihmen Reservoir)
context_page_description: Shihmen Dam is a major rock fill dam across the Dahan River in northern Taoyuan City. It forms the Shihmen Reservoir, Taiwan's third largest reservoir or artificial lake. It provides irrigation in Taoyuan, flood control for the Taipei Basin, and hydroelectricity and domestic water supply for more than three million people in northern Taiwan.
  Completed in 1964 after nine years of construction, Shihmen was Taiwan's first multi-purpose water project and a major step towards the island's economic independence after World War II. Year-round water releases from the dam enabled additional rice harvests and doubled Taoyuan's annual agricultural output, while the reservoir became a major tourist destination due to its scenic beauty and plentiful fisheries. However, the project was criticized for its high cost and its impact on local communities, as more than 2,000 people were displaced to make way for the reservoir.
  Like many other reservoirs in Taiwan, Shihmen has suffered from sedimentation, reducing its capacity by over a third. Efforts to reduce the rate of sediment accumulation, including dredging, check dam construction and watershed restoration work, have had a limited effect.
context_section_description: The large amount of sediment flowing into the reservoir has also caused unusually high nutrient levels, which leads to frequent eutrophication and algae blooms in the summer. As a result, Shihmen is often considered to be among the worst in water quality among all Taiwan's reservoirs. A large water-quality monitoring system is in place, installed circa 2005 at two points in Shihmen Reservoir. Called "Vertical Profiling System" or "YSI 6600EDS", the system measures a series of water quality factors by depth at intervals of 5 metres (16 ft). The turbidity, chlorophyll content, pH level, and dissolved oxygen levels in the reservoir are sent to the operator of the dam every three hours. A future plan for the system is to record algae growth, allowing action to be taken to prevent hypoxic events in the water. In 2008, the Taiwanese Environmental Protection Agency allocated NT$200 million for eutrophication control projects at Shihmen, Feitsui, Zengwun and Kinmen Reservoirs.
caption: Shihmen Reservoir (right) and afterbay (left), seen from the crest of Shihmen Dam.
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