Conowingo Dam
| Company type | Private |
|---|---|
| Industry | Hydroelectric power generation |
| Founded | 1926 |
| Founder | Philadelphia Electric Company (PECO) |
| Headquarters | Conowingo, Maryland , United States |
Area served | Mid-Atlantic region, primarily Philadelphia area |
Key people | Paul English (former dam superintendent) |
| Products | Hydroelectric power |
| Owner | Susquehanna Electric Company (subsidiary of Constellation Energy) |
Number of employees | 5,000 (construction period) |
| Parent | Constellation Energy |
The Conowingo Dam—also referred to as the Conowingo Hydroelectric Plant or Station—is a prominent hydroelectric dam situated on the lower stretches of the Susquehanna River near Conowingo, Maryland. Ranking among the largest hydroelectric dams in the United States outside federal ownership and standing as Maryland's biggest, this medium-height masonry gravity dam is a key contributor to power generation within the region. Positioned approximately from where the Susquehanna River empties into the Chesapeake Bay, it lies south of the Pennsylvania state line and northeast of Baltimore, Maryland, spanning Cecil and Harford counties.
The dam impounds a reservoir covering 9,000 acres, which submerged the original Conowingo townsite, subsequently relocated about northeast during the dam’s erection. The initial Conowingo Bridge, formerly part of U.S. Route 1, was dismantled in 1927 and replaced by a new crossing constructed atop the dam structure itself.
Recreational opportunities are supported by the Conowingo Reservoir and neighboring Susquehanna State Park, both of which enhance local economic benefits through tourism and energy production.
Construction and hydroelectric power generation
The site was historically known as Smyth's Falls and appears on a map created by John Smith in 1612.
On January 23, 1925, the Philadelphia Electric Company (PECO) awarded the contract for building the dam to Stone & Webster based in Boston, who also undertook its design. Construction commenced in 1926 under Maryland’s Arundel Corporation. Remnants of disused railroad tracks used for transporting materials are still visible along the river’s western bank. At its completion in 1928, this project was second only to Niagara Falls in size among U.S. hydroelectric developments.
Around 5,000 laborers converged on this rural northeastern Maryland location for construction activities. The scope extended beyond dam assembly to include railroad relocations, road paving, and installation of steel transmission towers directed toward Philadelphia. These efforts occurred almost five decades before federal workplace safety regulations were established, resulting in many workers encountering dangerous conditions and sustaining injuries.
A documentary produced by Maryland Public Television in 2016 highlighted concerns regarding fatalities during construction. Investigations confirmed at least fourteen deaths linked to work-related incidents during this period.
Originally designed with eleven turbine bays, seven turbines rated at 36 megawatts each were installed within a turbine house on the southwest side. An additional unit supplied 25 Hz electricity for the dam’s own electric railroad system, compatible with the electrified Pennsylvania Railroad line running along the eastern shore. In 1978, four more powerful turbines (65 megawatts each) were added in an open-air section at the northeast end of the dam, increasing total capacity from 252 to 548 megawatts. The generators produce electrical power at 13,800 volts, which is then stepped up to 220,000 volts for transmission primarily targeting Philadelphia. In 2021, this facility contributed approximately 2.1 million megawatt-hours to the electrical grid.
Currently managed by Susquehanna Electric Company—a subsidiary of Constellation Energy—the dam operates under a Federal Energy Regulatory Commission license renewed in 2021 that extends through 2071.
The Conowingo Hydroelectric Station also functions as a critical black start resource for restoring power within the regional PJM Interconnection grid during extensive outages.
Flood control
The dam is equipped with 53 flood control gates extending along most of its length beginning at the northeastern side of the powerhouse. These gates are operated using overhead cranes initially manufactured by Morgan Engineering Company, each with a specific load capacity. The cranes travel on rails installed atop the dam and are powered electrically. In 2006, a fourth crane featuring upgraded power rails was introduced. By 2017, all original cranes had been replaced by modern models that include backup generators to guarantee functionality during electrical outages.
The floodgates were fully opened for the first time in 1936. During Hurricane Agnes in June 1972, all 53 gates were again deployed as water levels neared a record high, exceeding usual reservoir levels by a significant margin. At this critical point, dam superintendent Paul English issued advisories regarding potential structural stability concerns. The dam experienced a record peak discharge and a downstream stream elevation that set new highs.
On January 20, 1996, the gauge recorded the second-largest crest accompanied by a substantial ice jam forming behind the dam. Conversely, the lowest discharge ever documented occurred on March 2, 1969.
During Tropical Storm Lee (2011) on September 9, 2011, 44 floodgates were opened as river levels reached the third-highest measurement on record. This event necessitated evacuation orders for Port Deposit, Maryland, situated approximately five miles downstream.
On July 26, 2018, heavy rainfall across the Mid-Atlantic region caused river levels to rise above critical thresholds, prompting the opening of 20 floodgates. This situation threatened flooding in nearby urban areas such as Port Deposit and raised environmental concerns about increased sediment and nutrient runoff into the Chesapeake Bay, potentially impacting water clarity and fostering algal bloom development. The United States Coast Guard issued alerts regarding debris fields entering the bay following gate operations.
Ecology and environmental impacts
The Conowingo Dam forms the 14-mile-long Conowingo Reservoir, which serves as a drinking water source for Baltimore and Chester Water Authority, provides cooling water for Peach Bottom Nuclear Generating Station, and supports recreational activities including boating and fishing. Balancing reservoir levels during drought conditions while ensuring adequate ecological flow downstream remains a crucial operational challenge.
Together with Holtwood and Safe Harbor Dams located upstream, Conowingo obstructs migratory fish species such as American shad from accessing their spawning habitats. To mitigate this, a fish lift was installed at Conowingo in 1991; by 2000, all three dams had fish passage facilities. That year, approximately 153,000 shad passed through Conowingo’s lift; however, only between 30% and 50% successfully continued upstream to Holtwood Dam, indicating persistent obstacles in migration.
These dams also act as barriers for sediment and nutrient transport from the Susquehanna watershed, thereby lessening their impact on the Chesapeake Bay ecosystem. A USGS study conducted in 1998 projected that reservoir capacity might be reached before 2020, which could reduce this natural filtration effect. Sediment accumulation rates are influenced by factors such as flood-induced scouring.
Bald eagles
Conowingo Dam represents one of the premier locations in the United States for observing bald eagles. While several dozen eagles reside in the area year-round, over 200 individuals migrate from regions including New York and Canada during winter months when frozen waters limit their fishing options. Eagle populations peak between November and January as they exploit fish stunned by the dam’s turbines for easier capture. This phenomenon attracts bird photographers worldwide and underscores the dam’s ecological importance as well as its role in promoting tourism.
See also
References
- Safe Harbor Water Power Corporation, Conestoga, PA. "Mechanics of Fish Lifts."
External links
- U.S. Geological Survey. Real-time flow data for Conowingo Dam
- Thinkquest (Oracle Education Foundation). "Conowingo Hydroelectric Facility" - Interactive tour featuring interior images and schematic diagrams
- National Weather Service. Flood stage information for Susquehanna River at Conowingo
- Maryland Public Television. Promotional Video: Conowingo Dam: Power on the Susquehanna [1] Overview of the MPT one-hour documentary
- Energy infrastructure completed in 1928
- Buildings and structures in Cecil County, Maryland
- Dams in Maryland
- Buildings and structures in Harford County, Maryland
- Hydroelectric power plants in Maryland
- Dams on the Susquehanna River
- Road bridges in Maryland
- U.S. Route 1
- United States power company dams
- Exelon
- Masonry dams
- 1928 establishments in Maryland
- Reservoirs in Maryland
- Reservoirs on the Susquehanna River
- United States privately owned dams
- Conowingo, Maryland