Correction: An earlier version of the article misstated the opening date of the Francis Scott Key Bridge, which spans the Potomac River between Georgetown and Rosslyn. The bridge opened in 1923, not 1924. This version has been corrected.

Graffiti and grass cover the trough that once held water for canal boats on the Potomac Aqueduct. The aqueduct was finished in 1843 and carried canal boats over the Potomac from the C&O Canal to the Alexandria Canal. (John Kelly/The Washington Post)

I walked over the Key Bridge recently and saw the remnants of the old C&O Canal aqueduct across the Potomac. That brought to mind several questions: When was the aqueduct built, how long was it used and when, how and why did it come down? Why would they go to the trouble and expense of creating an aqueduct instead of just using the Potomac?

— Glen Elliott, Reston

Let’s start at the beginning: Why did the C&O Canal need an aqueduct bridge across the Potomac? Actually, it didn’t. It was the Alexandria Canal that needed the bridge. When construction started on the C&O Canal in 1828 — it would eventually stretch 185 miles from Georgetown to Cumberland, Md. — Alexandria’s grandees realized their city’s port would suffer. Ships would simply bypass them and load goods at Georgetown’s wharves.

So they set about building a canal stretching from Rosslyn to Alexandria.

Although the C&O Canal did have access to the Potomac, a canal boat is not a riverboat. Using the Potomac would also mean running the risk of being hit by logs and other debris coming down the river, said Karen Gray, a volunteer historian at the C&O Canal. And what would you do with the mules that pulled the boat? It’s not as if they could clamp the tow ropes between their teeth and pull you across like magical dolphins.

Construction on the aqueduct bridge began in 1833 under the direction of Maj. William Turnbull of the U.S. Corps of Topographical Engineers. Also on the project was master carpenter Benjamin F. Miller, who was needed because the design called for a wooden superstructure to sit atop eight stone piers and two bridge abutments — a stone abutment on the Georgetown side, earth and stone on the Virginia side. (Of course, it wasn’t the Virginia side then. The land south of the Potomac wasn’t retroceded until 1846.)

It was an arduous project, beset by numerous problems. “A more difficult work has been rarely heretofore undertaken,” was how an annual report of the Topographical Bureau put it.

The bridge took 10 years to build, and when it was finished, it featured a 28-foot-wide white oak and North Carolina heart pine superstructure, with a trough 17 feet across and seven feet deep and a towpath five feet wide next to it. Canal boats traveled through what was essentially a 1,100-foot-long wooden box full of water.

Water is none-too-friendly to wood. To prevent rotting, all of the wood was “kyanized,” a process patented in 1832 by John H. Kyan, a Briton who discovered that impregnating wood with bichloride of mercury helped preserve it. (A kyanized piece of oak was said to have emerged unscathed after three years in the “fungus pit” maintained at England’s Woolwich dockyard to test preserving methods.)

The first canal boat reached Alexandria on Dec. 2, 1843. It was called the Pioneer and it bore executives of the canal company, the mayor of Alexandria and other dignitaries. The trip from Georgetown took a little more than an hour. The Alexandria Gazette expressed the hope that the canal and the Potomac Aqueduct would “RESTORE and PERPETUATE the TRADE and PROSPERITY of ALEXANDRIA.”

It did for a while. But over time, people decided that they needed a bridge for people and carts, not just boats. The Civil War prevented improvements, but in 1868 a second wooden level was erected over the first for a toll road. Later, it was replaced with an iron superstructure.

Despite the kyanizing, the wooden trough was prone to leaks, so many that in 1884 the president of the Potomac Boat Club complained that boaters couldn’t enjoy that stretch of river because they were perpetually showered with water from the sievelike aqueduct.

In 1886, the aqueduct bridge was closed to canal boats. By then, it was easier to pay a tug to take your barge from Georgetown to Alexandria. It continued as a bridge for traffic until the Key Bridge opened in 1923.

In 1933, the iron superstructure was removed. And then in 1962, seven of the eight piers were blasted away. The rubble was taken to Anacostia Park, where it was used for seawalls. Pier One — the one closest to the Virginia shore — was kept as a bit of history.

The two arches of the Georgetown abutment also remain. One shelters the boats of the Potomac Rowing Club. And at the foot of Montgomery Street in Alexandria is a restored lock from the otherwise-vanished Alexandria Canal.

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