Union City Reservoir is a unique part of a series of flood control projects in the Pittsburgh District Watershed. Located on French Creek among the rolling hills of Erie County, Pennsylvania, the Union City Reservoir is the only dry-bed reservoir in the district.
Authorized by the Flood Control Act of 1962, the dam was completed in 1971 and provides flood protection for the French Creek Valley, and to a lesser degree, the Allegheny River below Franklin, Pa. The reservoir has the capability of storing the equivalent runoff of 4.08 inches of precipitation from its 222 square mile watershed. The flood prevention benefits provided by the project since completion are estimated to be approximately $108 million.
Although Union City does not provide for typical lake-related activities, it does offer visitors the unusual opportunity to observe the process by which this dry bed reservoir becomes a lake.
The dam functions as an uncontrolled detention structure that automatically stores and releases water during periods of peak flow.
A 460-foot-long concrete conduit, 4.5 ft. by 8 ft., runs through the base of the dam to allow for uncontrolled discharge of waters. The conduit permits normal flows of French Creek to pass through unimpeded. When the amount of water entering the reservoir exceeds the dam’s capacity to discharge it through the drainage conduit, a temporary lake is formed.
If the flow of waters into the lake fills the reservoir to capacity the excess waters are then discharged into a side-channel spillway constructed through the right abutment of the dam.
The spillway was designed with a discharge capacity capable of handling the maximum flow of record at the damsite (14,900 cubic feet per second on April 5, 1947). At maximum flood pool Union City Reservoir will extend 7.4 miles up French Creek and cover 2,290 acres. As the water inflow decreases, the stored water gradually drains out, and once again the reservoir bed becomes dry.
The lake forming process usually occurs during late winter and early spring as a result of melting snow and spring rains. The resulting lake usually lasts only two or three months. However, if authorized, the present outlet facilities at Union City Reservoir could be modified to permit the creation of a summer pool of 580 acres.