Elizabeth Lock and Dam on Monongahela River, July 2023 (photo by USACE)

3 July 2024

What happens when a dam is removed from a river? We’re about to find out on the Monongahela River at Elizabeth, PA. On 10 July, weather and river permitting, the US Army Corp of Engineers (USACE) will begin removing Locks and Dam 3, also known as the Elizabeth Locks and Dam. Built in 1907 is one of the oldest such structures in the U.S.

This animation from USACE shows how it will be done.

Ideally the removal of a dam would restore the river to a more natural state but Pittsburgh’s three rivers are a series of pools — like lakes — created for navigation by numerous locks and dams.

schematic map of Locks and Dams on the Allegheny, Monongahela and Ohio Rivers near Pittsburgh, PA (original map from PA Fish & Boat Commission, markup by Kate St. John)

The dam at Elizabeth is 12.6 miles upstream from Locks and Dam 2 at Braddock and 23.8 miles downstream from Locks and Dam 4 at Charleroi. When the first gap is made in the Elizabeth dam it will take 3 days for the water level to stabilize into a pool 36.4 miles long stretching from Braddock to Charleroi. No, the Monongahela River will not revert to a natural state but there will be other effects.

As demolition continues on the rest of the dam, its removal will release sediments such as silt, clay, sand and gravel that have built up behind the dam for more than 100 years. Considering Pittsburgh’s industrial past, the sediments may contain toxins such as heavy metals and PCBs. My guess is that this will mostly affect the pool down to Braddock.

Eventually the turbidity will settle down and the Monongahela River will reach a new normal.

How much will the dam removal affect the Mon at Duck Hollow? We’ll have to watch and see.

Location of Locks and Dam 3 is on the Google map below. Read more about the project at The Waterways Journal.


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