Local History: The Old Conduit

While driving by the pop-up construction site for the “Old Conduit Repairs” across the bridge on my morning school run, I was reminded of some fascinating historical photos of the Washington Aqueduct. During daily traversing, it’s easy to forget that the Union Arch Bridge (the Cabin John Bridge) is on the National Register of Historic Places, a magnificent feat of engineering that was built to carry the Washington Aqueduct across the steep ravine and over Cabin John Creek. MacArthur Boulevard, previously known as Conduit Road, was never meant to be a busy thoroughfare but rather a service road for accessing and maintaining the massive underground pipe that lies beneath it.

Begun in 1857 by the U.S Army Corps of Engineers under chief engineer Lieutenant Montgomery C. Meigs, the Washington Aqueduct was designed to carry clean drinking water to Washington, D.C. The system was based upon the New York and Boston aqueducts completed the previous decade.

Constructed of brick, stone, and mortar, the underground conduit is nine feet in diameter. From the intake at Great Falls, where a dam was constructed in the Potomac River, the twelve mile-long circular pipe carries the diverted water to the Dalecarlia Reservoir. The aqueduct relies on gravity to direct the flow of water. From the reservoir, water is treated and distributed to some 1.1 million people in Washington, D.C., Arlington County and parts of Fairfax County. The aqueduct produces, on average, 135 million gallons of water per day. 

In order to carry the aqueduct across Cabin John Creek, Lieutenant Meigs designed the Union Arch Bridge. Completed in 1862, the Cabin John Bridge was the longest single span stone arch bridge in the world until 1903. Made in part with distinctive red Seneca stone, the bridge is 450 feet long with an arch span of 220 feet and was one of Meigs’s crowning achievements. His name is engraved in many locations along the aqueduct, including several places on the bridge.

Until 1923, the Old Conduit was the sole source of water for the Washington Aqueduct system. In the early 1920s, due to a growing city and higher demand for water, the Army Corps expanded the system and constructed a second conduit that runs parallel to the old one. This conduit’s centerline runs below the pedestrian/bike path along MacArthur Boulevard.  

In March 2022, an inspection of the Old Conduit revealed compromises to its structural integrity caused by violations of vehicle weight restrictions on the roadway. Repairs to the Washington Aqueduct system, in operation for an astonishing 160 years, will continue through the end of the year.

By Rachel Donnan, Contributing Writer

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