A three-part hydraulic engine
Songkhla Lake is not a single body of water, but a complex of lagoons covering 1,040 square kilometers, making it the largest natural lake in Thailand. The system consists of three main sections, each with a distinct identity. In the south, Thale Sap Songkhla connects to the Gulf of Thailand through a 380-meter-wide strait. This section is consistently brackish, with salinity levels that can reach up to 30 parts per thousand (ppt) in the dry season, close to that of seawater.
Moving north leads to the expansive and shallow Thale Luang, the largest part of the lagoon at 783 square kilometers. Its salinity is highly variable, influenced by both seasonal monsoons and tidal flows. During the rainy season from October to January, freshwater runoff can make this section almost entirely fresh. Farthest north lies the Thale Noi, a 28-square-kilometer freshwater marsh, protected as a Ramsar wetland of international importance.
Important to the lake's character is the twice-monthly spring tide. During these periods, denser, saltier water from the Gulf of Thailand pushes inland along the lake bottom. This intrusion forms a "salinity wedge," where the saltwater slides underneath the less dense freshwater flowing out towards the sea. This phenomenon creates a stratified water column, with salinity differences of about 3 ppt between the surface and the bottom at depths of only 2-3 meters.
A dynamic aquatic nursery
The constantly shifting salinity gradient makes Songkhla Lake a productive nursery ground for numerous aquatic species. The brackish water of Thale Sap and the fluctuating conditions in Thale Luang provide an ideal environment for the juvenile stages of commercially important prawns and fish. Post-larval prawns, including the banana prawn (Penaeus merguiensis) and the greasyback shrimp (Metapenaeus ensis), are carried into the lake system by the incoming saltwater tides. Here, they find abundant food and shelter among the mangroves and submerged vegetation, allowing them to grow rapidly.
Once they mature, these prawns migrate back to the open ocean to spawn, completing a life cycle that depends entirely on the lake's connection to the sea. The lake also supports a major aquaculture industry for Asian sea bass (Lates calcarifer), which are grown in net cages within the brackish southern waters.
This delicate hydrological balance is under threat. Pollution from surrounding agriculture, shrimp farms, and urban areas like Hat Yai introduces excess nutrients and contaminants into the water. Siltation from land erosion is also making the already shallow lake—with an average depth of just 1.4 meters—even shallower, altering its hydraulic properties and harming aquatic life.