The hydrology system of Thua Thien Hue province is very sophisticated and unique.
This feature is reflected in the dense river system including O Lau River - Tam Giang lagoon - Huong River - Loi Nong River - Dai Giang River - Ha Ta River - Cong Quan River - Truoi River - Nong River - Cau Hai lagoon. Before reaching the sea, majority of rivers except for A Sap and Bu Lu meet in a large lagoon stretching nearly 70km along the sea. It is called Tam Giang - Cau Hai, the most typical lagoon in Vietnam, the largest lagoon in Southeast Asia, and one of the largest lagoons in the world.
This river-lagoon system is connected with many natural lagoons as well as big and small man-made dams. The total water surface area of Tam Giang - Cau Hai lagoon is about 231km. The total volume of water running from the east of Truong Son range is over 9 billion m3.
According to Thua Thien Hue Geography - Nature chapter
(Social science publishing house - 2005)