The topography of Korea is generally high and mountainous along the east coast, and low with more subdued relief along the west coast. Consequently, the longest rivers flow into the Yellow Sea and the Korea Strait. The shoreline of the east coast is relatively smooth, and rivers flowing into the East Sea are short with steep gradients. Rivers flowing to the more complex, irregular western and southern coasts tend to be long, with gentle gradients, meandering channels, and higher flow volumes. These rivers have deposited extensive and fertile alluvial plains. 

 

 In Korea, there are five major rivers: Hangang, Nakdonggang, Geumgang, Seomjingang, and Yeongsangang. The Han River (Hangang) has the largest drainage area of 35,770 km² (including the portion in North Korea), and an annual discharge of 16 billion m³, constituting 35% of the nation's total runoff volume. The longest river in Korea is Nakdonggang, with a length of 510 km. 

 

 Given the long history of agriculture in Korea, some stream systems reflect efforts by humans to manage and control water for irrigation, flood mitigation, and other purposes. Rice was first introduced to Korea’s alluvial soils in the Neolithic Age and became widespread throughout the south during the Bronze Age. Samguk sagi, the first historic record to mention rice farming in Korea, documents that King Daru established rice paddies across the countryside in 33 A.D. As rice farming grew, nationwide projects were conducted to build structures such as embankments, waterways, and reservoirs that would facilitate the access and storage of water. 

 

 From 2005 to 2014, the average annual precipitation in Korea was 1,323 mm, which is equivalent to 1.6 times the world average. This is enough to classify Korea as a high rainfall region, although seasonal variability is extremely high. Due to monsoonal rain and typhoons, 736 mm of rainfall (56% of the annual rainfall) is concentrated during the summer and often causes floods. Furthermore, rainfall has the tendency to quickly collect in rivers, as over 70% of the land is mountainous with an average slope of about 20%. These geomorphological and climatic characteristics cause high fluctuations in the flow rate of rivers throughout the year, which can cause both floods and droughts.