Columbia Island is in part natural, and in part man-made. About 1818, Analostan Island (now known as
Theodore Roosevelt Island) was largely rock and quite close to the D.C. shoreline. Due to deforestation and increased agricultural use upstream, the river eroded much of the northern bank of the Potomac River and widened the gap between Analostan Island and the shore. Simultaneously, large deposits of silt built up around Analostan Island. By 1838, Analostan had almost doubled in length toward the south. By 1884, the new southern part of Analostan Island was defined and built up, and supported a well-established
wetland. Gradually, however, the river eroded the center of Analostan Island, severing Columbia Island from its parent body. Between 1911 and 1922, the Potomac River was repeatedly dredged by the
United States Army Corps of Engineers to deepen the channel and alleviate flooding.
Dredging widened the distance between Analostan/Theodore Roosevelt Island and Columbia Island (so that the "Virginia Channel" west of Analostan/Roosevelt Island would not flood easily). Dredged material was piled high on Columbia Island, helping to build it higher, lengthen and broaden it, and give it its current shape. An anonymous Corps of Engineers officer named the waterway between Columbia Island and Virginia the "Boundary Channel". Boundary Channel was further defined in the late 1920s. In 1925, Congress authorized construction of the
Arlington Memorial Bridge across the Potomac River. Preliminary designs for the bridge showed it terminating on Columbia Island, which necessitated additional expansion of the island. The Corps of Engineers had already planned to continue dredging the Potomac River and enlarge Columbia Island, so on April 1, 1925,
Secretary of War John W. Weeks ordered the expenditure of $114,500 to dredge the river between the
Highway Bridge and the
Lincoln Memorial. The dredged material was dumped on Columbia Island. By June 30, 1927, dredging of the Potomac River and the reshaping of Columbia Island was largely finished. Boundary Channel marks the original Virginia shoreline. It separates Columbia Island to the east from Virginia in the west and south, and is roughly a mile long. The channel is part of the Potomac River. In 1936, Boundary Channel was in width and in depth. No lagoon existed at this time. Boundary Channel was changed again after ground was broken for
The Pentagon on September 11, 1941. The Pentagon was being built on land just south of the Boundary Channel. But the ground to the northwest, north, northeast, and east of the building site was so low that, for a time, the Corps of Engineers considered building a
levee to protect it from floods. But
General Brehon B. Somervell, the Army officer in charge of the Pentagon's construction, decided instead to raise the ground by at least to or more above the average water level. Boundary Channel was dredged and slightly widened in order to help provide this fill material. Due to silting and other issues, Boundary Channel is approximately wide as of 2013 (although the width varies). It is also quite shallow. ==Pentagon Lagoon==