Approximately 30 percent of the area of South Korea consists of lowlands, with the rest consisting of uplands and mountains. The great majority of the lowland area lies along the coasts, particularly the west coast, and along the major rivers. The most important lowlands are the Han River plain around Seoul, the Pyongt'aek coastal plain southwest of Seoul, the Kum River basin, the Naktong River basin, and the Yongsan and the Honam plains in the southwest. A narrow littoral plain extends along the east coast.

The Naktong is South Korea's longest river (521 kilometers). The Han River, which flows through Seoul, is 514 kilometers long, and the Kum River is 401 kilometers long. Other major rivers include the Imjin, which flows through both North Korea and South Korea and forms an estuary with the Han River; the Pukhan, a tributary of the Han that also flows out of North Korea; and the Somjin. The major rivers flow north to south or east to west and empty into the Yellow Sea or the Korea Strait. They tend to be broad and shallow and to have wide seasonal variations in water flow.

News that North Korea was constructing a huge multipurpose dam at the base of Mount Kumgang (1,638 meters) north of the DMZ caused considerable consternation in South Korea during the mid1980s . South Korean authorities feared that once completed, a sudden release of the dam's waters into the Pukhan River during north-south hostilities could flood Seoul and paralyze the capital region. During 1987 the Kumgang-san Dam was a major issue that Seoul sought to raise in talks with P'yongyang. Though Seoul completed a "Peace Dam" on the Pukhan River to counteract the potential threat of P'yongyang's dam project before the 1988 Olympics, the North Korean project apparently still was in its initial stages of construction in 1990.