Gillespie Dam

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Gillespie Dam
Gillespie Dam.jpg
The Gila River flows freely through the breached dam.
Official name Gillespie Dam
Location Maricopa County, Arizona, USA
Opening date 1921[1]
Dam and spillways
Impounds Gila River
Height 20 feet (6.1 m)
Length 1,700 feet (520 m)
Reservoir
Creates Gillespie Dam Reservoir (former)

The Gillespie Dam is a concrete gravity dam located on the Gila River between the towns of Buckeye and Gila Bend, Arizona. The dam was constructed during the 1920s for primarily irrigation purposes. A portion of the dam failed unexpectedly in 1993 during unusually heavy rains.

History[edit]

The Gillespie Dam was constructed in 1921 by a local rancher to replace an existing structure.[2] As the dam was located at an important river crossing that would later become US Route 80, the Arizona Highway Department - the predecessor to the Arizona Department of Transportation - constructed a concrete apron at the foot of the dam to allow for vehicular crossings. As the dam was a simple spillover construction, during times of heavy runoff cars would have to be pulled through the flow by trucks, and during floods could not cross at all.[1]

In anticipation of the formation of the United States Highway System in 1926, the Highway Department commissioned the construction of a steel truss bridge just downstream from the dam. The bridge was completed and opened to traffic on August 1, 1927 at a cost of US$320,000 (US$3,950,000 in 2007). The bridge, which was at the time the longest highway bridge in the state of Arizona, was immediately incorporated into the highway system as Route 80.[1] The bridge carried US 80 traffic until 1956 when the highway was decommissioned, devolving to a county highway, thus placing the bridge under Maricopa County care. The bridge was added to the National Register of Historic Places on May 5, 1981.[3]

Failure[edit]

The winter months of 1993 saw unusually high rainfall amounts that resulted in record flows through central Arizona rivers and streams, including the Salt River, a major tributary to the Gila upstream from the Gillespie Dam.[4] At approximately 10:30 on the morning of January 9, the dam failed when a segment approximately 120 feet (37 m) in length collapsed into the river. While the precise cause of the failure is unknown, the extreme flooding was almost certainly a contributing factor. The precise size of the flood was not recorded due to equipment failure, but an estimate based upon a high-water mark recorded on USGS equipment yielded a peak flow of approximately 200,000 cubic feet (5,700 m3) per second, corresponding to a predicted 65-year flood, or a flood of a magnitude anticipated only once per 65 years. The previous high, recorded during similarly disastrous floods in 1980, had been 178,000 cubic feet (5,000 m3) per second.[5]

Due to the failure, three underground natural gas lines were exposed and later severed by the floodwaters. The bridge downstream survived, and was deemed safe for travel.[1]

The remnants of the dam remain in place and the area is largely accessible to the public. A small earthen embankment exists to divert water into nearby canals.

References[edit]

  1. ^ a b c d "Historic Gillespie Dam". Town of Gila Bend. Retrieved 2008-06-03. [dead link]
  2. ^ "Field Excursion Reports". United Nations University. Retrieved 2008-06-03. 
  3. ^ "Gillespie Dam Bridge". Arizona State Parks. Retrieved 2008-06-03. [dead link]
  4. ^ "Arizona's Most Notable Storms". National Weather Service. Retrieved 2008-06-03. 
  5. ^ Larson, Robert; James Slosson (1997). Storm-induced Geologic Hazards: Case Histories from the 1992-1993 Winter. Geological Society of America. pp. 32–33. 

Coordinates: 33°13′45″N 112°46′10″W / 33.22917°N 112.76944°W / 33.22917; -112.76944