Building a bridge across the Potomac was not an easy task. The river is 18 feet deep at this location, and construction had to go down through another 17 feet of silt before they could reach bedrock.

A set of (leaky) cofferdams were built that allowed workers to climb down 35 feet below the water surface. Boats would then lower massive stones down the hole to build up the piers. Once the piers were finished, a wooden trough was built on top of them. This was filled with water and linked the C&O Canal with the Alexandria Canal. Construction took ten years.

Civil War Era

During the Civil War the aqueduct was drained and boarded over to form a military roadway. This bridge provided a critical logistical connection to the defensive forts and batteries guarding the heights around Rosslyn. Civil War Washington was protected by a ring of such forts (called the circle forts).

After the war the government returned the bridge to the Alexandria Railroad and Bridge Company. They left the roadway intact until 1866, when it was turned back into an aqueduct.

The post-war aqueduct had major leakage issues. The Washington Post complained in 1885 that

"[the aqueduct's] present condition is very obnoxious to all persons who are fond of boating, for there is not a single arch under which they can go without receiving a drenching from the continual downpouring of water from the leaks above."

Metal Bridge