These stone walls by the Washington Monument aren’t a part of any memorial or historic structure. They’re part of a temporary dam that can span 17th Street and hold back a 100-year flood. The dam is the newest part of the 80-year-old Potomac Park levee.

Washington’s Federal Triangle neighborhood is uniquely vulnerable to flooding. First, the area is built on low lying land; if the river crests East Potomac Park it would quickly rush downhill and inundate the cross streets off Constitution Avenue. The second threat comes from rain runoff; Federal Triangle is the lowest point in a drainage basin stretching from the ballpark to Fort Totten.

The stakes are even higher because of what’s in those Federal Triangle buildings. Flood damage to the National Archives, EPA, IRS and Department of Justice could be devastating.

The original Potomac Park Levee was a temporary structure built during a flood in 1936 to keep rising waters from reaching downtown. That weather event spurred Congress to authorize the construction of a larger earthen barrier and make the levee permanent. The Army Corps of Engineers completed the project in 1939. Most people walking along the sloping footpaths around the tidal basin don’t realize that they’re standing on manmade berms that were carefully laid out to prevent a watery disaster.

17th Street was the only gap in the levee—a deliberate but foolish engineering decision. The gap provided a pleasant even grade for automobile drivers traveling north to south, and it centralized the risk of a flood, so during a storm engineers could focus their efforts on a single preplanned fail-safe rather than fighting the waters at a thousand different locations along the levee.

In the event of flood conditions, the plan called for a hastily built sandbag wall to plug the gap. Unfortunately, in the case of the floods of 1937, 1942, 1972, and 2006, that wasn’t good enough.

Washington finally moved to address the issue with construction of the present-day stone and cement structure. The gap in the center can be filled in with posts and panels that are capable of holding back waters up to 19 feet above sea level. They’re stored ready to go on trucks in Bladensburg and can be assembled in a few hours. The crews practice once a year to keep up their readiness and check the equipment.