In this paper the author first presents an original procedure to assess the impact of sea level rise. Following this procedure the natural and socioeconomic characteristics of the Songkhla Lake are described. Then the lacustrine lowland of Thale Sap Songkhla is divided into three distinctive geographical zones, and major factors that control the development of each littoral zone are identified. Based on those development factors, some serious impacts of sea level rise on the lake were assessed as follows ;
(1) In the beach ridge plain, coastal erosion will become more severely, especially where large-scale shrimp farming developed will affected critically by the retreat of the shoreline along the present coast.
(2) The area of Songkhla City should be protected by higher or stronger seawall if the coastal erosion would become more severe.
(3) The channel linked the lake and the Gulf of Thailand will become twice or three times as wide as present one, because of the erosion of the north end of the spit. Then the groundwater in the littoral lowlands will be affected by the increase in salinity of the lake water.
(4) In the deltaic lowland of Thale Sap Songkhla, wide littoral area will be inundated with lake water about 1 to 3 km in width from the present lake shoreline. Lacustrine lowlands newly covered with urban facilities, should be protected from severe floods or long-term inundation.