Gillespie Dam

The Gillespie Dam was constructed circa 1920 by a local rancher, Frank Gillespie, to replace an existing structure.[2][3] As the dam was located at an important river crossing that would later become U.S. 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 the Gillespie Dam Bridge, 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.[4]

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.[5] 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.[6]

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.