Shimen Dam

Shihmen Dam 石門水壩
View from the top of the dam, showing the spillway (left) and Shihmen reservoir.
Country	Taiwan
Location	Fuxing, Taoyuan City
Coordinates	24°48′38″N 121°14′39″E / 24.81056°N 121.24417°E / 24.81056; 121.24417Coordinates: 24°48′38″N 121°14′39″E / 24.81056°N 121.24417°E / 24.81056; 121.24417
Construction began	July 1955
Opening date	June 14, 1964
Construction cost	NT$1.4 billion
Owner(s)	Taiwan Water Resources Agency
Dam and spillways
Type of dam	Rolled rockfill
Impounds	Dahan River
Height	133.1 m (437 ft)
Length	360 m (1,180 ft)
Spillway type	Gated overflow
Spillway capacity	11,400 m3/s (400,000 cu ft/s)
Reservoir
Creates	Shihmen Reservoir
Total capacity	309,120 dam3 (250,610 acre·ft) (nominal) 251,880 dam3 (204,200 acre·ft) (1997)
Catchment area	763.4 km2 (294.8 sq mi)
Surface area	8.0 km2 (2,000 acres)
Power station
Turbines	2x 45 MW
Installed capacity	90 MW
Annual generation	200 million KWh

Shihmen Dam (Chinese: 石門水壩; literally: "Stone Gate Dam"; also spelled Shimen or Shihman) is a major rock fill dam across the Dahan River in northern Taoyuan City. It forms the Shihmen Reservoir (石門水庫), 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. This has diminished both the Shihmen Dam's capability to contain floods and provide water during droughts.

The Dahan River is the main river in northwestern Taiwan and is one of two tributaries that join in Taipei to form the Tamsui, Taiwan's largest river system. Originating in the Xueshan Range, the river begins life as a fast-flowing mountain stream prone to flooding during typhoons. The Dahan skirts, but does not cross, the Taoyuan Plateau, which lacks a reliable water supply of its own. The plateau has been farmed since 1680 using thousands of artificial ponds to capture rainfall; in 1928, when Taiwan was under Japanese rule, the Taoyuan Canal was dug to divert water from the Dahan (then known as the Takekan River) onto the plateau. River water enabled the irrigation of an additional 20,000 hectares (49,000 acres) of land in the Taoyuan area. Due to the limited storage capacity of the small Taoyuan ponds, there was often not enough water for irrigation in the dry season of November–April, demonstrating the need for a large reservoir.