South Korea's geography: a mountain spine that turns every river west

Where the Taebaek Mountains run

South Korea is mostly mountain. The Geography of South Korea article puts mountains at three-quarters of the land and lowland at about 30%, and early European visitors compared the country to "the sea in a heavy gale" for the way one ridge follows another. The rock underneath is old: Britannica describes it as largely Precambrian granite and gneiss, more than about 540 million years old.

The Taebaek Mountains run roughly north to south along the eastern coastline and continue into North Korea, and they form the country's drainage divide. Ranges branch off them to the south-west, and the most important, the Sobaek Mountains, bends across the peninsula in a long S-shape. Britannica calls none of the mountains very high, giving 1,708 metres for Mount Seorak in the Taebaek and 1,915 metres for Mount Jiri in the Sobaek. Along the east coast only a narrow plain lies between the Taebaek and the sea.

The peninsula as a whole covers 223,170 square kilometres including its islands, and the Geography of South Korea article gives the Republic of Korea 44.8% of that, 100,210 square kilometres, not counting the area inside the Demilitarized Zone. Japan's islands of Honshū and Kyūshū lie about 200 kilometres to the south-east across the Korea Strait, and China's Shandong Peninsula 190 kilometres to the west. The constitution claims more than the state administers: Article 3 defines the territory as the whole Korean peninsula and its adjacent islands.

Because the spine hugs the east coast, the country's long slope runs the other way, down towards the Yellow Sea and the Korea Strait, and the plains and the rice sit on that western and southern side. So does Seoul, whose metropolitan area holds about half the population.

Why nearly every river flows west or south

The three principal rivers, the Han, the Geum and the Nakdong, all rise in the Taebaek and run between the ranges before they reach their plains. Nearly every river in the country flows westward or southward into the Yellow Sea or the East China Sea; only a few short, swift streams drop eastward off the Taebaek.

The Nakdong is the longest. The Geography of South Korea article gives it 521 kilometres and Britannica 523, running south to the Korea Strait. The Han, 514 kilometres long, passes through Seoul, and the Geum runs 401 kilometres. The rivers are broad and shallow, and their flow swings with the seasons, high in the wet summer and far lower in the dry winter.

Two rivers cross the border. The Imjin flows through both Koreas and meets the Han in a shared estuary, and the Bukhan, a tributary of the Han, flows south out of North Korea. When news arrived in the mid-1980s that the North was building a large dam at the foot of Geumgangsan, 1,638 metres high, north of the zone, Seoul built a Peace Dam on the Bukhan to counter it before the 1988 Olympics.

The lowlands lie along those rivers and along the western coast: the Han plain around Seoul, the Pyeongtaek coastal plain to its south-west, the Geum and Nakdong basins, and the Yeongsan and Honam plains of the south-west. The south-western rice country is also where serious droughts fall hardest, about once every eight years. Land-use figures for 2011 put arable land at 15.3% of the country and permanent crops at 2.2%, and the country is self-sufficient in rice and potatoes.

How the western coast breaks into islands and mud

The two coasts are different kinds of shoreline. The east coast is relatively straight. The west and south are creek-indented ria coasts with long inlets and many islands, and the difference comes from the land itself, which is slowly rising along the east and subsiding along the south and west. About 3,000 islands lie off the western and southern coasts, the great majority small and uninhabited.

The Yellow Sea is shallow, and against that indented coast it produces a large tide. Britannica gives a maximum range of about 9 metres at Incheon, the port for Seoul. The mud the tide exposes is extensive: a global remote-sensing analysis cited in the Geography of South Korea article found 1,833 square kilometres of tidal flats. UNESCO inscribed the Getbol, Korean Tidal Flats on the World Heritage List in 2021, and its record of the site carries a second date, 2026.

Much of that mud has been reclaimed. Work at Saemangeum, Shiwa, Songdo, Namyang Bay, Asan Bay, Gwangyang Bay and the Nakdong estuary has destroyed wetland habitat, and the Geography of South Korea article records large declines in fisheries and in biodiversity as the result.

What the volcanic islands add

Two islands were made by volcanoes, and both sit apart from the granite mainland. Jeju, about 100 kilometres off the southern coast, has a land area of 1,848 square kilometres and is built around Hallasan, the country's highest point at 1,950 metres in both Britannica and the Geography of South Korea article. Hallasan is dormant; it has not erupted in many centuries, and Goryeo-era records describe volcanic activity on it. UNESCO listed the Jeju Volcanic Island and Lava Tubes as a World Heritage site in 2007. Ulleung lies about 140 kilometres east of the mainland in the sea South Korea calls the East Sea, and a small lava plateau in Gangwon province completes the volcanic record.

The rest of the peninsula is geologically quiet by comparison with Japan. There are no active volcanoes in South Korea and earthquakes are infrequent, though two since 2016 have exceeded magnitude 5.4.

About 85 kilometres south-east of Ulleung lies a group of rocky islets, called Dokdo in Korean, Takeshima in Japanese and the Liancourt Rocks in English. South Korea occupies and claims them, and Japan claims them too. This catalogue records the dispute and takes no position on it.

What the monsoon does to the year

The climate is set by the Asian mainland more than by the Pacific. In winter, cold dry air from the continent gives a long, dry winter with monthly averages below freezing everywhere except the southern coast; Britannica puts the January average at about minus 5 °C in Seoul and about 2 °C in Busan. Summer is short, hot and humid and far more even across the country, with August, the warmest month, averaging about 25 °C.

Rain arrives with the summer monsoon. The short rainy season called jangma begins at the end of June and lasts through July, and Britannica puts up to three-fifths of annual precipitation in June, July and August. Seoul shows the pattern at its sharpest: over the 1981 to 2010 normals its annual mean was 12.5 °C and 63% of its precipitation, 892 millimetres, fell from June to August, while humidity peaks at 80 to 90% in July and drops below 50% in winter and spring.

Jangma is less a steady rain than a stalled front. On average it lasts 30 to 35 days, with rain on only about 12 to 16 of them, often as sudden downpours at night, and it brings more than 30% of the peninsula's precipitation, which is why some call it a fifth season. A shorter second rainy season can return in late August and early September as the Pacific high pressure retreats. In 2022 the head of the Korea Meteorological Administration said the recent heavy rain had been impossible to predict and that the traditional idea of a rainy season now seemed to have expired.

Typhoons come late in the summer, usually one to three a year and mostly in August, bringing heavy rain to the southern coast. In September 1984 floods killed 190 people and left 200,000 homeless, and South Korea accepted rice, medicine and building materials offered by North Korea.

In spring, yellow dust blown from the deserts of China and Mongolia is observed on three to six days a year, mainly from March to May.

How the climate is moving north

The Korea Meteorological Administration measures a steady rise. Its analysis published in June 2026, using 66 weather stations, found the national annual average temperature had risen by 0.3 °C a decade over 1973 to 2025, and the average for 2025 was 13.7 °C. The Geography of South Korea article records 2024 at 14.5 °C.

The same analysis found about 80% of the country still classed as temperate, with only the seven months from April to October averaging above 10 °C. Over thirty-year averages, fourteen places already meet the subtropical definition, among them Mokpo, Wando, Tongyeong, Geoje and Jeju; over the most recent ten years, Gwangju, Uljin and Gangneung qualify as well. Under a high-emissions scenario the KMA projects that by 2081 to 2100 most of the country, all but the inland west of Gangwon province, will be subtropical. Wildfires have become more frequent and more intense, and a fire in North Gyeongsang in 2025 burned 99,289 hectares.

How the forests grew back

The mountains were not always wooded. Forests were cut heavily in the first half of the twentieth century and during and after the Korean War, and the loss brought flooding and soil erosion. Forest cover fell to a low of 35% of the land in 1955.

Recovery was a matter of policy. Arbor Day became a national holiday in 1949, firewood was barred from entering Seoul and other cities from 1958, and comprehensive planting programmes ran from the 1970s into the late 1990s. Forest cover peaked at 65% in 1980, and land-use figures for 2011 put forest at 63.9%. Most of the soil beneath those forests comes from the same granite and gneiss: sandy, brown, well leached and low in humus, with ash-grey podzolic soils in the highlands where winters are long. About 4,500 plant species are known and some 380 bird species, most of them seasonal migrants. Britannica notes that little of the original forest survives: pines are the characteristic trees, with evergreen broad-leaved camellias and camphor trees confined to a narrow subtropical belt along the southern coast and on Jeju.

What grew back in the Demilitarized Zone

The one land border is the Demilitarized Zone, a strip about 4 kilometres wide that runs 241 kilometres from coast to coast along the armistice line of 1953, 238 of them on land. Farmland before the war and a battleground during it, the zone has lain almost undisturbed since the fighting stopped.

Britannica describes it as a de facto nature preserve. Its forests, estuaries and wetlands shelter hundreds of bird species, among them the endangered white-naped and red-crowned cranes, as well as dozens of fish species, Asiatic black bears and lynxes. Beyond the zone the country has 20 national parks, Jirisan among them, and natural areas such as Suncheon Bay Ecological Park. Across the rest of the country tigers, leopards, lynx and bears have almost disappeared, and deer are the most numerous large mammals.