Thac Ba Lake is the reservoir of the dam of the same name. Its course disrupted, the Chay River flooded 20 000km² and over a length of 80km the whole eastern part of Yen Bai province, forming one of the two largest lakes of Upper Tonkin. The 1 330 or so islets scattered about the surface are the tops of red earth hills that are now submerged. The lakeside inhabitants (the Dao and Tay peole) of what is called the "Halong Bay of the mountains" live from fish farming and prawn fishing.