How Does the National Mall Tidal Basin Actually Work?

By Special Contributor posted 11-07-2019 14:13


Before we can save something, we must understand it. Even as a fresh graduate of a landscape architecture program, I was surprised by the design and operation of the National Mall Tidal Basin.

I knew about the Tidal Basin before working on the campaign to save it at the National Trust for Historic Preservation. Like many visitors to Washington, D.C., I saw that the reservoir links the Thomas Jefferson, Martin Luther King Jr., Franklin Delano Roosevelt and other monuments. The well-known cherry blossoms wrap the Basin’s rim. Beyond its aesthetic and commemorative roles, however, one of the most interesting features of the Tidal Basin is its infrastructure.


Blue zone indicating new land of West and East Potomac Park | Credit:  Map from Library of Congress. Hains, Peter C, J. E Hilgard, and U.S. Coast and Geodetic Survey. Proposed improvement of James Creek Canal. [Washington, D.C.: United States Coast and Geodetic Survey, 1887] Map. 

The Tidal Basin did not exist 150 years ago. The land was formed with soil and sediment dredged from the Potomac River. Sediment was in the Potomac due to erosion from mining, timber logging, and charcoal production upstream.  Deforested areas did not retain the soil, and during rain events unrooted sediment washed into the Potomac. The legacy of the Tidal Basin would not be what it is today without inland industries.

What was once water is now land, and the ground we stand on today at the Tidal Basin may not always be a given. Inspired by a visionary history, the future of the Tidal Basin has tremendous potential.


Transformation of the Potomac River. | Credit: Historic maps from Library of Congress. 

Though originally called Twining Lake, the Tidal Basin is not a natural lake. Rather, in the 1880s, it was envisioned and later designed and constructed (1882-1909) as an engineering invention that leverages the power of tides of the Potomac River. The Tidal Basin was intended and continues to clear the Washington Channel allowing boats to navigate along the Southwest neighborhoods of D.C. The Inlet and Outlet Bridges on the Tidal Basin Reservoir house sets of automatically swinging tidal gates. Gate design drawings are available through the Library of Congress.

To understand the daily workings of the Tidal Basin, I sketched, animated, and made a “CliffsNotes” style outline to better understand its functioning:

Summary of flows into and out of the National Mall Tidal Basin. | Credit: Phia Sennett


  • During the Potomac River’s low tides, the inlet gate automatically closes, and the outlet gate opens.
  • As the tide rises, water floods into the Tidal Basin in the inlet.
  • Water is captured by the Basin because the outlet gate closes concurrently.
  • The Basin then experiences high tide.
  • When the tide begins to fall, water in the Tidal Basin flows back into the Potomac River.
  • The inlet gates shut with changing currents.
  • Water can only exit from the outlet gate and through the Washington Channel. The sudden flush of water clears sediment at the bottom of the channel, maintaining the appropriate depth necessary for boats to navigate.

Bird's-eye picture of the city of Washington, view of the Tidal Basin. | Credit: Washingtoniana Map Collection (1916)

The result is human design working in equilibrium with natural rhythms.

A landform created from mud harnesses tides and automates its own maintenance. The Tidal Basin is a landscape that sculpts itself.


The Tidal Basin has operated automatically (without motorized control) since the U.S. Army Corps of Engineers designed and oversaw completion of this project. The Tidal Basin was completed in its entirety in 1911, a year before the cherry trees were gifted from the People of Japan to the People of the United States.


Man with easel and group of photographers at the Tidal Basin facing the Washington Monument in the background, with cherry trees in blossom, Washington, D.C. Between 1909 and 1932. | Credit: National Photo Company Collection, Library of Congress, Washington, D.C.

The cherry blossoms are easy to read. They are at our human scale — more approachable than the monumental size of other memorials on the National Mall. Their beauty is instant and instinctively understood. Perhaps this is why the cherry trees have become a personal backdrop for important life moments experienced at the Tidal Basin: picnics, quinceañeras, weddings, reunions, school trips, and graduations.

The infrastructural mechanisms of the Tidal Basin are more hidden and ask us to take different vantage points to fully understand.

The Tidal Basin Ideas Lab, presented by American Express, launched on October 22 with a visit by the five design firms to Washington, DC.  The National Trust, with their partner, the Trust for the National Mall recently announced the names of those firms, which are DLANDStudio, Hood Design Studio, GGN Landscape Architects, James Corner Field Operations, and Reed Hilderbrand.  These firms will soon begin a working period to develop bold, innovative solutions for the Tidal Basin. 


Challenges at the Tidal Basin. | Credit: Phia Sennett

The design firms will be tasked with proposing innovative and bold ideas about the numerous challenges the Tidal Basin is facing. Challenges become most obvious twice a day during high tide, when pathways around the Tidal Basin are inundated with water. Visitors must navigate flooded sidewalks and the jersey barriers temporarily protecting this landmark. Maintenance demands, physical security, sea level rise, and stresses from millions of annual visitors (and their footprints) compromise the Tidal Basin experience. For these reasons the Tidal Basin was listed as one of the National Trust for Historic Preservation’s 2019 America’s 11 Most Endangered Historic Places.

The Tidal Basin plays an important role in the nation’s capital and requires investment and imagination.

A key insight I have taken away from the months spent at the National Trust for Historic Preservation is that preservation is a process, and the story does not stop at the site’s boundary line.

Phia Sennett is a landscape architect who interned in the Summer of 2019 at the National Trust for Historic Preservation. Want more information on the National Mall Tidal Basin? Sign up for updates and follow #SaveTheTidalBasin. 


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