The fascinating story of DC’s aqueducts and reservoirs

Construction of the Washington Aqueduct Image by Titian Peale/Public Domain.

Have you ever wondered where the District's drinking water comes from, or puzzled over why the McMillan reservoir is located in the center of the city, far from the water sources that supply it? GGWash contributor Elliot Carter's DC Underground Atlas showcases the history and engineering of this hidden underground world. Here's a taste of what he found.

Washington's serious water problem

Like many growing American cities in the mid-19th Century, 1850s Washington had a water problem. The city is located at the confluence of the Potomac and the Anacostia, but these are tidal rivers that are too salty to be used for drinking. DC's original water sources were wells and natural springs, but these limited water sources couldn't provide for the city's growing needs, nor supply the pressure needed to fight fires, especially on the upper floors of buildings.

After an 1851 fire at the Library of Congress destroyed two-thirds of the collection inside, Congress decided that DC's limited water supply was a problem of national importance. Congress instructed the Army Corps of Engineers to find an “unfailing and abundant supply of good wholesome water” and to build an aqueduct to meet the city’s needs.

Steps in a vault in the Washington Aqueduct system, with engineer Montgomery Meigs's name engraved on them. Image by Elliot Carter used with permission.

Enter Montgomery Meigs, a Corps of Engineers officer and, later, the Union Quartermaster General throughout the Civil War. Meigs was put in charge of the Washington Aqueduct project in 1852. A brilliant engineer and administrator, he was also a self-promoter who saw the project as an opportunity to build a water supply for the capital that would last "for the next thousand years."

Meigs considered a number of water sources within the District, but decided that even Rock Creek, with its daily flow of eight million gallons, was insufficient for the growing city. Instead, he decided to build a diversion dam on the Potomac at Great Falls, near the current site of the Great Falls Tavern Visitor Center. This location was far above the salt and urban sewage of the tidal Potomac and had a rapid flow of 67 million gallons a day to feed the aqueduct.

Building the Washington Aqueduct

With a site selected, Meigs was faced with the much larger challenge of getting water from Great Falls to downtown Washington. The site of the diversion dam was 10 miles northwest of the District boundary, and it was another eight miles to Capitol Hill. Furthermore, before the dam and aqueduct could be built, the Corps had to cut a road along the Potomac from Georgetown to Great Falls to allow access to the construction sites.

Underground conduits, nine feet in diameter and lined with three layers of brick, were built alongside and under the roadway from Great Falls to the Dalecarlia Reservoir on the Maryland-District border. Much of the dirt service road Meigs built to facilitate the construction of the aqueduct — originally named Conduit Road — was eventually paved and became today's MacArthur Boulevard.

MacArthur Boulevard is now a major commuting route, albeit with a six-ton vehicle weight limit to protect the water tunnels under the median. However, the northernmost mile and a half of the old service road runs within C&O Canal National Historic Park. This portion of the road remains in its original unpaved state as the "Berma Road" hiking trail on the bluffs along the inland side of the canal from near Great Falls Tavern to the Old Anglers Inn parking lot.

By designing the aqueduct to rely entirely on gravity to provide water flow and building most of it as underground tunnels, Meigs ensured that it would require little maintenance over the centuries he hoped it to last. However, the valleys of small streams feeding the Potomac forced the water conduits above ground in several places. Most notably, the 100-foot-deep valley of Cabin John Creek was spanned by the Union Arch Bridge, which was the longest single-arch masonry bridge in the world from 1864 to 1903.

Although the Union Arch Bridge was originally built only to carry the water conduit, a roadbed was eventually built on it for MacArthur Boulevard. However, since the bridge was only built wide enough for the conduit, traffic is limited to a single lane with a signal to alternate the direction of traffic flow.

The Dalecarlia and Georgetown Reservoirs and the city tunnel

Water from the Washington Aqueduct flows into the Delacarlia Reservoir on the DC-Maryland border. When the aqueduct was originally built, no water filtration was planned. Instead, water was supposed to sit in the Delacarlia Reservoir long enough for sediment to settle out of it before flowing downhill to the Georgetown Reservoir. From there it would be distributed to the Georgetown and Washington water mains.

However, the fast flow through the system — which didn't have enough storage capacity to allow long dwell times in the reservoirs — didn't give sediment time to settle. When the system first opened in the 1860s, there were numerous complaints about the color and flavor of the water. The Georgetown Reservoir was relatively low-lying, and did not supply sufficient water pressure to the top floors of buildings in higher-elevation parts of the District.

In 1882, Congress addressed the situation by agreeing to fund a four-mile tunnel deep-bored through bedrock from the Georgetown Reservoir to a location near the Howard University campus, where a new, larger reservoir would be built on higher ground. Garrett Lydecker, a major in the Army Corps of Engineers, was put in charge of building this new tunnel.

Inspection of the City Tunnel while it was under construction. Image by Army Corps of Engineers.

Under Lydecker's direction, the project ran into major delays and cost overruns. Some of the work was shoddy, and they discovered that the bedrock through which the tunnel was bored was too fractured to provide a water-tight seal (the same problem leading to water infiltration along the Red Line in Northwest today). The tunnel eventually had to be lined with three layers of brick to provide a seal for water, with cast-iron walls to handle the high water pressures at the tunnel's deepest point under Rock Creek.

A Congressional investigation in 1888 into the tunnel's high costs and slow construction discovered embezzlement and fraud, and led to Lydecker's court-martial for mismanagement. Construction was abandoned until 1898, when Senator James McMillan — chair of the Senate Committe on the Distract of Columbia and better known for his McMillan Plan for Washington's monumental core — pushed through funds for its completion.

The McMillan reservoir and modern water filtration

When the City Tunnel finally opened in 1901, water from Georgetown Reservoir flowed to the new McMillan Reservoir on the heights near Howard University. However, when the reservoir opened in 1902, it became clear that the added reservoir capacity was still not sufficient to remove all of the sediment from the District's water supply. The McMillan Reservoir Filtration Plant, an enormous slow sand filtration site, opened in 1905 and removed both sediment and a significant fraction of the bacteria from the water leaving the reservoir.

To supplement sand filtration, the city began adding chlorine to kill bacteria in the water in 1923. The system was expanded in 1927 to handle the District's increasing need for water as well as the needs of Arlington and part of Fairfax County, including the present-day City of Falls Church. This meant building a second parallel tunnel from Dalecarlia to Great Falls and adding a fast sand filtration plant adjacent to the Dalecarlia Reservoir to purify water for the Virginia suburbs and the western portion of the District.

The towers of the McMillan Sand Filtration Plant, viewed through the fencing surrounding the site while its ultimate fate is decided. Image by Flickr user bootbearwdc licensed under Creative Commons.

Finally in the 1950s, the system was expanded for a final time with a second diversion dam and water intake at Little Falls near the Dalecarlia Reservoir. In addition, the McMillan Reservoir Filtration Plant was replaced with a smaller and more modern fast sand filtration plant in 1985, beginning a decades-long process of planning a redevelopment of the site.

Today, the Washington Aqueduct system is still owned and operated by the Army Corps of Engineers. It acts as a wholesaler, selling water to the DC Water and Sewer Authority and Arlington and Fairfax Counties.