Alexandria Tide Lock Park
GPS Coordinates: 38.8137796, -77.0382548
Closest Address: 66 Canal Center Plaza, Alexandria, VA 22314

Here follows an excerpt from the Atlas Obscura website:
Alexandria Tide Lock Park
Alexandria, Virginia
Long buried under the 20th-century landscape, this lift lock of the Alexandria Canal is the lone remnant of an ambitious early American transportation project.
With growing settlement east of the Appalachian Mountains and into the Ohio Valley, a rapidly expanding United States needed better and more efficient ways to transport raw goods to the business centers on the Atlantic and finished goods to the growing frontier. Canal works sprung up where feasible, most notably the Erie Canal from Albany, New York to Buffalo, and the Chesapeake & Ohio Canal from the Georgetown neighborhood of Washington D.C. to Cumberland, Maryland.
Alexandria city merchants, sensing a loss of business to its upstream competitor, pushed forward with what was then a truly ambitious engineering project for the early 19th century. Their solution was a seven-mile canal starting in Georgetown, running south through Alexandria County, and ending at the north end of the Alexandria Potomac waterfront. Begun in 1830 and completed in 1843, the Alexandria Canal included two aqueducts, one over the Potomac from Georgetown to Rosslyn, and the other crossing Four Mile Run in northern Alexandria. Three lift locks and a final tide pool lock in Alexandria, where waiting ships transferred goods, dropped barge traffic 38 feet from Georgetown.
During its 40 years of operation, barges using the Alexandria Canal completed the “last mile” for grain, lumber, and particularly coal from the Piedmont to ocean-going ships. Finished goods started their journey to the American frontier from here. While very successful, construction saddled Alexandria with considerable debt, and operating the canal was always expensive. Accelerated by an expanding railroad network and demand for a toll-free bridge across the Potomac, use and profitability declined after the Civil War and never recovered. Operations ceased in 1886.
Most of the original canal has been buried under 20th-century construction in Arlington County and Alexandria City. The City of Alexandria began archeological work on the four lift locks lying within its city limits in the late 1970s and through the next decade. Plaques mark the approximate locations of the first three locks in northern Alexandria. Today Tidal Pool and Lift Lock No. 1, and its lock mechanism, are re-created and publicly accessible at Alexandria Tide Lock Park, located at 1 Canal Center Plaza.
Private developers worked closely with the archeological and interpretive teams to give the recreation an attractive, modern look while bringing to life the narrative and interpretation of the engineering artifacts. Visitors can walk around the complex and enjoy a lunch, take a rest if they are cycling the on the adjacent Mt. Vernon bike trail, or simply take in the views of the Potomac River rolling by.
Know Before You Go
Alexandria Tide Lock Park is at 1 Canal Center Plaza in North Old Town Alexandria, VA. Easiest access is on foot or bicycle on the Mt. Vernon Trail, where the park is just north of Oronoco Bay Park. The Park lies on private land but is readily accessible to visitors.
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Here follows an excerpt from the Clio Foundation website about the Alexandria Canal Tide Lock as written by Ben M.:
Introduction:
This lock is the only remnant left of the Alexandria Canal, which was completed in 1843 and operated until 1886. The canal began here and ran north, crossing the Potomac River on an aqueduct to the Georgetown neighborhood of Washington D.C. The canal connected Alexandria to the Chesapeake and Ohio Canal (C&O), which was located in Georgetown and ran inland to Cumberland, Maryland. This canal was used primarily to transport coal, an enterprise that was essential to Alexandria's economic growth. The lock was added to the National Register of Historic Places in 1980.
Backstory and Context:
Calls for an Alexandria canal came on the heels of the construction of the C&O canal, which was finished in 1831. As a result, ocean-going ships tended to bypass Alexandria and go straight to Georgetown, which threatened the city's growth. Construction of the canal and aqueduct began in 1834 and were completed ten years later. Some of the coal transported here was used by local industries, which helped spur industrialization. The canal thus played a critical role in Alexandria's economic development.
By the 1880s, railroads had emerged as the dominant form of transportation in the country. Use of the Alexandria Canal ended in 1886 and over time it was filled in and an electronic trolley line was built along part of the towpath of the canal. The remnants of the aqueduct's structure were removed in 1923. The tide lock was also filled in but it was restored in the 20th century as part of a project to revitalize the city's waterfront.