For the dry half of the year the lake is a modest thing - two hundred hectares of surface, about four metres deep, sitting quietly among rice fields roughly thirty-five kilometres up the road from Châu Đốc. Then the monsoon arrives, the Mekong swells, and water pushes in through the connecting channels until the lake covers eight hundred hectares and runs eight metres deep. It quadruples and it doubles, and it does this every single year. The lake links the Bình Di River to the Hậu, and it touches three communes - Khánh Bình, Nhơn Hội and Quốc Thái - so the flood does not merely raise the water level; it redraws where the shoreline is and which fields are now lake bed.

The clarity has a mechanism, and it is entirely biological. Water hyacinth, lotus, water lily, algae and submerged weed cover and line the basin, and their combined root mass does three jobs at once. It traps and holds incoming silt before the suspended load can spread. It oxygenates the water column. And it pins the lake bed down so that flood surges passing through cannot stir the sediment back up into the water. A river-fed basin that should be as opaque as the Mekong itself instead behaves like a settling pond with a permanent green filter across the top. Local science has taken enough interest in this to publish on the sediment characteristics of the basin, which is a very unglamorous way of describing something people have found miraculous for two centuries.

The story people tell is that a general of the Tây Sơn chose this spot in the eighteenth century to drill troops and store grain, and found it barren and waterless. A ritual was held, petitioning the gods for rain. The general drove his sword into the earth, and where the blade went in, water came up, and kept coming, and became the lake. Almost the identical legend attaches to the Gia Long Well on Phú Quốc, where the sword belonged to Nguyễn Ánh - the man who destroyed the Tây Sơn and took the throne as Emperor Gia Long. Two mortal enemies, the same miracle, a hundred and fifty kilometres apart. Whatever else the story is doing, it is not being partisan about who deserves to find water.

The villages around the lake are Cham. Roughly twelve thousand Cham live in this district, concentrated in five settlements including Nhơn Hội, Khánh Bình and Quốc Thái, and they are Sunni Muslim - a distinct tradition from the Bani Cham of central Vietnam. Their presence here dates largely to the mid-nineteenth century, when Cham Muslim families moved down from Cambodia and settled the country around Châu Đốc. The mosque at Nhơn Hội, Masjid Khoy Ri Yah, stands close enough to the water to be reflected in it. Cham households in An Phú still weave cloth on traditional looms, and many people here move between Cham, Vietnamese, Khmer and Malay - a practical inheritance for a community that has lived on a trading river with a border across it for a very long time.

This is the far northern tip of the Vietnamese delta, and Cambodia is close on more than one side; the Bình Di River, which the lake connects to, is itself part of the international boundary, and the crossing at Long Bình opens onto the Cambodian market town of Chrey Thum. The administrative geography changed underneath the lake in the summer of 2025. Vietnam dissolved its district tier nationwide, so An Phú district ceased to exist, and An Giang province absorbed neighbouring Kiên Giang, with the enlarged province now run from Rạch Giá. The lake will quadruple again in June, as it always does, and the Cham villages on its edge will still be where they were.