Shimen Dam (Chinese: 石門水壩; literally "Stonegate Dam"; anglicized as Shihmen or Shihman) is an embankment dam crossing the Dahan River in Taoyuan County, Taiwan. Serving mainly for municipal water supply and flood control, the dam creates Shimen Reservoir in the mountains south of Longtan. The construction plan was created in 1938 under Japanese rule, but was not implemented immediately because of the start of World War II. The dam was the largest in Taiwan when construction ended in 1964.
Each day, Shimen supplies 1.4 million tonnes of water to residences and industry and 1.8 million tonnes of water to agriculture in Taoyuan County and New Taipei City. It is integral to the water supply/regulation system of northern Taiwan. The dam cost NT $4.85 billion to construct.
The dam site lies in a steep canyon of the Dahan (大漢) River near the aboriginal town of Fusing, at the head of a 763 km2 (295 sq mi) catchment area. The canyon, with walls up to 500 metres (1,600 ft) high, was formerly home to the summer villa of Chiang K'ai-shek. The historic arched Amuping Stone Bridge and a nearby Earth God shrine, among other landmarks, were also covered by the Shimen Reservoir as it filled. Before the dam was built in the 1950s, the flow of the Dahan River below the dam was copious enough in the rainy season to make it navigable from the coast to points in Taipei and Tamsui.
In August 2004, Typhoon Aere hit northern Taiwan, dumping 973 millimetres (38.3 in) of rain on the region. The resultant erosion caused by rainfall caused landslides to carry up to 20 million tons of debris into the Shimen Reservoir. Although turbidity levels in the reservoir usually measure roughly 40 NTU and the water filtration plant can handle up to 3,000 NTU, it was clogged by the sediment in the water which reached a peak of 70,000-120,000 NTU. As a result, homes, businesses, and farms that relied on Shimen Reservoir had their water supply cut off. For several days, these customers had to subsist on trucked-in water.