A large, brackish, coastal lagoon, about 20 km long and up to three km wide. The lagoon extends in a northwest-southeast direction parallel to the coast, and is separated from the sea by a sandy ridge formed by coastal deposition. The northern end of the lagoon is the mouth of O Loan River; both the river and the lagoon open into the sea at the Thuan An and Tu Hien channels.
Thua Thien-Hue Province plain along the sea coast has many lagoons, such as those of Tam Giang, Thuy Tu, Lap An, into them are converged the water of almost all the rivers of the province of Thua Thien Hue before going to the sea, through the mouths of Thuan An, Tu Hien, or through the great lagoon of Lang Co.

Tam Giang and Cau Hai are the 2 biggest lagoons with fresh water, the 2 most representative lagoons in Vietnam with a length of 70km for the whole of the two, they form an ideal habitant for many sea species all along the province.

The lagoon supports a major fishery which is important not only as a protein source for the local communities, but also for export. The annual production of finfish alone is estimated at 3,600 metric tonnes. The lagoon also has considerable potential for outdoor recreation and tourism.

A very important area for wildlife. The lagoon supports a rich fish and invertebrate fauna, and is a major staging and wintering area for migratory waterfowl, particularly ducks and shorebirds.

Tropical monsoonal climate with a rainy season from August to December; the heaviest rainfall occurs in October and November. The mean January temperature is 20.1°C, and the mean June temperature 29.2°C.

Yearly thousands tons of sea products are brought on here (fish, shrimps and others). In the recent years, the cultivation and raising of sea products, mainly the raising of shrimps and cultivation of "gracilaria" has developed in this zone of lagoons.