The Conowingo Dam (also Conowingo Hydroelectric Plant, Conowingo Hydroelectric Station) is a large hydroelectric dam in the lower

Susquehanna River The Susquehanna River (; Lenape: Siskëwahane) is a major river located in the Mid-Atlantic region of the United States, overlapping between the lower Northeast and the Upland South. At long, it is the longest river on the East Coast of the ...

near the town of Conowingo,

Maryland Maryland ( ) is a U.S. state, state in the Mid-Atlantic (United States), Mid-Atlantic region of the United States. It shares borders with Virginia, West Virginia, and the District of Columbia to its south and west; Pennsylvania to its north; ...

. The medium-height, masonry gravity dam is one of the largest non-federal hydroelectric dams in the U.S., and the largest dam in the state of Maryland. The dam sits about from the

river mouth A river mouth is where a river flows into a larger body of water, such as another river, a lake/reservoir, a bay/ gulf, a sea, or an ocean. At the river mouth, sediments are often deposited due to the slowing of the current reducing the carryin ...

at the

Chesapeake Bay The Chesapeake Bay ( ) is the largest estuary in the United States. The Bay is located in the Mid-Atlantic (United States), Mid-Atlantic region and is primarily separated from the Atlantic Ocean by the Delmarva Peninsula (including the parts: the ...

, south of the

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border and northeast of

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, on the border between Cecil and Harford counties. The dam supports a 9,000-acre reservoir, which today covers the original town of Conowingo. During dam construction, the town was moved to its present location about northeast of the dam's eastern end. The rising water also would have covered

Conowingo Bridge Several incarnations of the Conowingo Bridge crossed the Susquehanna River at the original location of Conowingo, Maryland, United States, about two miles upstream of the Conowingo Dam, which replaced it. History The original Conowingo Bridg ...

, the original U.S. Route 1 crossing, so it was demolished in 1927. U.S. Route 1 now crosses over the top of the dam. The Conowingo Reservoir, and the nearby Susquehanna State Park, provide many recreational opportunities.

Construction and hydroelectric power generation

The area, which appears on a 1612 map by

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, was originally known as Smyth's Falls. On January 23, 1925, Philadelphia Electric Company awarded the construction contract for the dam to Stone & Webster of

Boston Boston (), officially the City of Boston, is the capital city, state capital and List of municipalities in Massachusetts, most populous city of the Commonwealth (U.S. state), Commonwealth of Massachusetts, as well as the cultural and financ ...

, who did the design. Construction, which started in 1926, was carried out by the Arundel Corporation of Maryland. (Abandoned railroad tracks for transporting heavy equipment to the dam site can be seen along the western shore of the river below the dam.) When completed in 1928, it was the second-largest hydroelectric project by power output in the United States after

Niagara Falls Niagara Falls () is a group of three waterfalls at the southern end of Niagara Gorge, spanning the border between the province of Ontario in Canada and the state of New York in the United States. The largest of the three is Horseshoe Fall ...

. Some 5,000 workers flocked to this rural northeastern corner of Maryland, seeking to earn good pay as construction got underway. In addition to those working directly on the dam, large numbers relocated railroad tracks, paved new roads, and constructed steel towers to stretch the heavy transmission lines toward Philadelphia. This was nearly fifty years before Congress passed the Occupational Safety and Health Act, which guaranteed the right to a safe job. While these men struggled to earn a living, many suffered disabling injuries handling high voltage electric lines, tumbling from high elevations, managing explosives, encountering venomous snakes, etc. When Maryland Public Television aired its documentary, "Conowingo Dam: Power on the Susquehanna" for Chesapeake Bay Week in April 2016, the question came up about how many workers died performing their duties. While investigating the death of Hunter H. Bettis on November 26, 1927, Darlington Coroner Wiliam B. Selse commented in ''

The Baltimore Sun ''The Baltimore Sun'' is the largest general-circulation daily newspaper based in the U.S. state of Maryland and provides coverage of local and regional news, events, issues, people, and industries. Founded in 1837, it is currently owned by Tr ...

'' that more than twenty men had lost their lives. In an attempt to create a registry or census of job-related fatalities, death certificates, newspaper accounts, and funeral home books were examined. Fourteen individuals were identified. The dam was built with 11 turbine sites, although only 7 turbines were initially installed, driving generators each rated for 36 megawatts. A turbine house, on the southwestern end of the dam, encloses these seven units. One additional "house" unit provides 25 Hz power for the dam's electric railroad system (identical to that used by the

Pennsylvania Railroad The Pennsylvania Railroad (reporting mark PRR), legal name The Pennsylvania Railroad Company also known as the "Pennsy", was an American Class I railroad that was established in 1846 and headquartered in Philadelphia, Pennsylvania. It was named ...

, which had an electrified line ow under Norfolk Southern ownership">Norfolk_Southern.html" ;"title="ow under Norfolk Southern">ow under Norfolk Southern ownershiprunning on the eastern shore). In 1978, four higher-capacity turbines were added. Each of these turbines drives a 65-megawatt generator, increasing the dam's electrical output capacity from 252 to 548 megawatts. The four newer turbines are in the open air section at the northeast end of the powerhouse. The generators produce power at 13,800 volts. This is stepped up to 220,000 volts for transmission, primarily to the Philadelphia area. The dam currently contributes an average of 1,600 Megawatt-hours annually to the electrical grid. Through subsidiaries and mergers, the dam is now operated by the Susquehanna Electric Company, which is part of Constellation Energy. The current Federal Energy Regulatory Commission license for the dam was issued in 2021 and expires in 2071. The Conowingo Hydroelectric Station would be a primary black start power source if the regional PJM power grid ever had a widespread emergency shutdown ( blackout).

Flood control

The dam has 53 flood control gates, starting at the northeastern end of the powerhouse and spanning the majority of the dam. The floodgates are operated by three overhead cranes rated for each which were originally built by the Morgan Engineering Company of Ohio. The cranes run on rails the length of the dam and are electrically powered from lines that run above the face of the dam. A 4th crane was installed in 2006 at the powerhouse end of the rails, which required installing new power rails below the existing power wires. By 2017, all original overhead cranes had been replaced. The original Morgan cranes were removed and scrapped. The new cranes have backup generators installed on them to allow operation during a facility grid power failure. In 1936, all the floodgates were opened for the first time. During

Hurricane Agnes Hurricane Agnes in 1972 was the costliest hurricane to hit the United States at the time, causing an estimated $2.1 billion in damage. The hurricane's death toll was 128. The effects of Agnes were widespread, from the Caribbean to Canada, ...

, in June 1972, all 53 floodgates were opened, for only the second time, and explosives planted to blow a section of the

weir A weir or low head dam is a barrier across the width of a river that alters the flow characteristics of water and usually results in a change in the height of the river level. Weirs are also used to control the flow of water for outlets of l ...

, as the waters rose during the early morning hours of June 24 within of topping the dam (a record crest of , above normal level for the entire long Conowingo Reservoir.) Paul English, dam superintendent, released a bulletin at 10:30 p.m. on June 23 saying that the water levels were reaching a point at which the stability of the dam "cannot be controlled. When it reaches ...it will be in the hazy area...It is not known...whether a structure of the dam may give and...people downstream should be advised." At this time, the flow sensor in the dam recorded its record discharge of , and the stream height gauge, at the dam's downstream side, registered a record . On January 20, 1996, the gauge recorded its second-highest recorded crest of . A severe ice jam also developed behind the dam on this date. The record minimum recorded discharge was on March 2, 1969, when the flow sensor registered . On September 9, 2011, 44 flood gates were opened due to the impact of the remnants of

Tropical Storm Lee The name Lee has been used for seven tropical cyclones worldwide. In the Atlantic, it replaced the name '' Lenny''. In the Atlantic: * Tropical Storm Lee (2005), a short-lived, minimal tropical storm * Tropical Storm Lee (2011), a strong tropical ...

. The Susquehanna River level behind the dam was , the third-highest in history. The town of Port Deposit, located 5 miles southeast of the dam, was evacuated. On July 26, 2018, 20 of the 53 floodgates were opened due to rising floodwaters resulting from several days of torrential downpours in the Mid-Atlantic. The Susquehanna River saw water levels of over , placing nearby cities, like Port Deposit, at risk of flooding like in 2011. The ecological impact of this event is not known, but

ecologists are concerned about the health of the Chesapeake Bay with the influx of sediment and limiting nutrients, such as phosphorus and nitrogen, impacting water clarity and promoting

algal bloom An algal bloom or algae bloom is a rapid increase or accumulation in the population of algae in freshwater or marine water systems. It is often recognized by the discoloration in the water from the algae's pigments. The term ''algae'' encompas ...

s. The

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issued warnings for all vessels in the Chesapeake Bay regarding the fields of debris, floating and submerged, that had been released when the floodgates opened.

Ecology and environmental impacts

The river water impounded by the dam forms the long Conowingo Reservoir, known locally as Conowingo Lake. The reservoir is used as a

drinking water Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, ...

supply for

and the Chester Water Authority; as cooling water for the

Peach Bottom Nuclear Generating Station Peach Bottom Atomic Power Station, a nuclear power plant, is located southeast of Harrisburg in Peach Bottom Township, York County, Pennsylvania, on the Susquehanna River three miles north of the Maryland border. The Philadelphia Electric C ...

; and for recreational boating and fishing. In low rainfall or

drought A drought is defined as drier than normal conditions.Douville, H., K. Raghavan, J. Renwick, R.P. Allan, P.A. Arias, M. Barlow, R. Cerezo-Mota, A. Cherchi, T.Y. Gan, J. Gergis, D. Jiang, A. Khan, W. Pokam Mba, D. Rosenfeld, J. Tierney, an ...

conditions, balancing the desire to maintain the reservoir level with the water flow needs for the downstream ecology is one of the challenges faced by the dam operators. The Conowingo Dam, and to a lesser extent the Holtwood and Safe Harbor Dams further upstream, stopped migratory fish species, especially American shad, from swimming further up the Susquehanna River to spawn. In 1984, a fish capture feature was added at Conowingo and shad were trucked upstream above all three dams and released. This program ended in 1999. A

fish lift A fish ladder, also known as a fishway, fish pass, fish steps, or fish cannon is a structure on or around artificial and natural barriers (such as dams, locks and waterfalls) to facilitate diadromous fishes' natural migration as well as mo ...

was installed in 1991. All three dams completed installation of

s in time for the 2000 season. During the 2000 migration season, 153,000 American shad passed through the Conowingo fish lift. "However, passage rates of shad from Conowingo to Holtwood have been only 30 to 50 percent, suggesting that fish are having difficulty moving upstream in the waters of the Conowingo pool." The three dams are also involved with another ecological concern. Normally, the reservoirs above each dam trap sediment and nutrients that run off from the Susquehanna River watershed and prevent some of that from reaching the Chesapeake Bay. A 1998

USGS The United States Geological Survey (USGS), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

study suggests that the reservoirs may reach capacity before 2020, and cease to reduce the nutrient and sediment load hitting the bay. However, the scouring of major floods, and other factors, affect this.

References

* Safe Harbor Water Power Corporation, Conestoga, PA
"How Fish Lifts Work."

External links

* * U.S. Geological Survey
Conowingo Dam real-time water flow data
* Thinkquest (Oracle Education Foundation)

- Virtual tour with interior pictures and diagrams * National Weather Service
NWS Flood Stage Info. for Susquehanna River at Conowingo
* Maryland Public Television. Promo Video: Conowingo Dam: Power on the Susquehann

An introduction to the MPT one-hour documentary {{authority control 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 Conowingo Conowingo United States privately owned dams

Construction and hydroelectric power generation

Flood control

Ecology and environmental impacts

See also

References

External links