The historic Gillespie Dam Bridge marker. The historic Gillespie Dam Bridge was built in 1927 over the Gila River and is located on Old Highway 80 north of Gila Bend and south of Arlington between the Buckeye Hills and the Gila Bend Mountains in Maricopa County. The bridge was at the time the longest highway bridge in the state of Arizona. The bridge was listed on the National Register of Historic Places on May 5, 1981, reference #81000136