The problem was distance. Repair ships, self-propelled barracks ships and tank landing ships had moved into the Gulf of Siam in December 1968, but they sat far too far off to keep the riverine patrols supplied inland. Admiral Elmo Zumwalt's answer was to put the logistics where the fighting was: an Ammi pontoon complex moored mid-river across from Năm Căn, towed down by dock landing ships to the mouth of the Bồ Đề and hauled the rest of the way by tugs. It grew to thirteen barges — formally a mobile advanced tactical support base — offering berths, meals and stores to American Patrol Craft Fast, riverine assault craft and gunboats, to South Vietnamese Navy vessels, and to SEAL detachments. A Huey gunship from the HA(L)-3 squadron flew off a landing platform bolted to one of the barges.

Bristling with the guns of the moored combat craft plus emplaced mortars and automatic weapons, Sea Float was, in the phrase the Navy's own historians used, a defensive porcupine. Its best protection was not ordnance at all but hydrology. The Cửa Lớn runs at six to eight knots as the tide drives water between two seas, and a swimmer carrying explosives against that current has a very bad night ahead of him. Sappers tried anyway. In April 1970 guards broke up an attack by swimmers equipped with Soviet-made underwater gear, killing four of them with grenades and rifle fire. That July, mines opened a hole in the repair ship USS Krishna as she lay at anchor in the river, though she stayed afloat. Later the same month a mine capsized the South Vietnamese landing ship LSSL-225. Seventeen of her sailors died — men from a navy whose losses in these creeks are rarely counted anywhere outside their own families.

Năm Căn itself had been a fishing and charcoal-cutting village on swampy ground, poor and unremarkable, until it was overrun during the Tet Offensive and left almost entirely destroyed. Its few old roads were abandoned and impassable, and until 1969 the area was effectively conceded to the Viet Cong, as the American and South Vietnamese commands called the southern insurgency. In November 1969 General Creighton Abrams endorsed Zumwalt's push to plant a permanent government presence here under the name Operation Solid Anchor, and the resources followed. US Air Force aircraft sprayed Agent Orange across the country around Năm Căn and the barges to strip the enemy of cover — the same mangrove that had been the villagers' fuel, their building timber and their fishery. As the fighting eased, civilians began drifting back to a place that had been altered in ways nobody was measuring yet.

Moving ashore was harder than staying afloat. The Seabees found that the sodden sand simply would not carry a modern naval base — not the buildings, not the heavy plant needed to put them up. A contractor took over the dredging and filling, and by 1970 the calculation had reached 640,000 cubic yards of fill, three to five feet of it spread over nearly the whole site, plus a strip for aircraft. What eventually stood on that manufactured ground was twelve Butler buildings, forty-one prefabricated SEA huts, a fleet of thirty-nine assorted patrol boats and a population of roughly a thousand, including housing the Seabees built for South Vietnamese sailors' families. An airfield with a short asphalt runway sat barely two metres above sea level.

In September 1970 the shore base was ready, Sea Float was disestablished, and the Navy towed its barges away to other work. Control of Solid Anchor passed to the Republic of Vietnam Navy in April 1971, by which point construction had run to fifty million US dollars; the RVNN's 5th Coastal Flotilla operated twenty-seven patrol craft from it. American advisers stayed until February 1973, leaving a month before the last US military personnel departed South Vietnam. The base ended in 1975 along with the country it had been built to hold together. Today Năm Căn is a working port town of shrimp ponds and crab boats, and the river that once defended a floating fortress simply carries the tide back and forth as it always did.