Lake Songkhla, a coastal lagoon produced by sand-bar formation includes three lakes, Thale Noi (2,800 ha), Thale Luang (78,280 ha) and Thale Sap Songkhla (17,600 ha), from north to south, which are inter-connected by narrow channels. A narrow strait (minimum width 380 m) connects Thale Sap with the sea (Gulf of Thailand) at its southeastern end. A grandient of salinity exists, therefore, between the brackish water of Thale Sap and the pure freshwater of Thale Noi. The middle lake, Thale Luang, approaches a freshwater condition during the rainy season (October - January), but is influenced by the invasion of seawater in other month.

The lakes are shallow throughout and moderately eutrophic, with a mean water depth of 1.2 -1.3 m in relatively dry seasons, but the water level rises by about one meter during the winter months. Thale Sap is known for its production of fish, shrimp and crabs, and is also intensively utilized for the aquaculture of seabass (Lates calcarifer). The fry produced and supplied by the National Institute of Coastal Aquaculture in Songkhla is grown in net cages by fishermen families. Thale Noi and the eastern branch of Thale Luang, called Kukut, have been designated as an area for waterfowl protection.

The catchment area of about 8,020 km2 consistes mostly of lowland rice fields, rubber plantations and forest - covered hills, and contains such rapidly growing cities as Hat Yai and Songkhla. The waste water from certain manufacturing and factories has caused local pollution of the lake and some damage to the fisheries, but the complete and rapid turnover of water in the rainy season seems to prevent severe problems (Q, 1, 2).