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. In the early part of the 20th century and especially the period during and after World War II and the Korean War, much of the existing Korean forests were cut down, which led to problems with flooding and soil erosion. Combination of reforestation efforts (e.g. Arbor day was celebrated as a national holiday starting in 1949) and policies designed to reduce the use of firewood as a source of energy (e.g. restriction of inflow of firewood into Seoul and other major cities starting in 1958) helped to spark a recovery in the 1950s. Comprehensive reforestation programs starting in the 1970s and continuing into the late 1990s aided in an acceleration of forest volume increase, and the forest cover reached a peak of 65% of national land area in 1980 as opposed to a low of 35% in 1955. News that North Korea was constructing a huge multipurpose dam at the base of Geumgangsan () north of the DMZ caused considerable consternation in South Korea during the mid-1980s. 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 Geumgangsan Dam was a major issue that Seoul sought to raise in talks with Pyongyang. Though Seoul completed a "Peace Dam" on the Pukhan River to counteract the potential threat of Pyongyang's dam project before the 1988 Olympics, the North Korean project still was in its initial stages of construction in 1990. Maritime claims: territorial sea: ; between and in the Korea Strait contiguous zone: exclusive economic zone: continental shelf: not specified Elevation extremes: lowest point: Sea level 0 m highest point: Hallasan Climate Part of the East Asian Monsoon region, South Korea has a temperate climate with four distinct seasons. The movement of air masses from the Asian continent exerts a greater influence on South Korea's weather than does air movement from the Pacific Ocean. Winters are usually long, cold, and dry, whereas summers are short, hot, and humid. Spring and autumn are pleasant but short in duration. Seoul's mean temperature in January is ;