The mountain ranges in the northern and eastern parts of North Korea form the watershed for most of its rivers, which run in a westerly direction and empty into the Yellow Sea (Korea Bay). The longest is the Yalu River, which is navigable for 678 of its 790 kilometers. The Tumen River, one of the few major rivers to flow into the Sea of Japan, is the second longest at 521 kilometers but is navigable for only 85 kilometers because of the mountainous topography. The third longest river, the Taedong River, flows through P'yongyang and is navigable for 245 of its 397 kilometers. Lakes tend to be small because of the lack of glacial activity and the stability of the earth's crust in the region. Unlike neighboring Japan or northern China, North Korea experiences few severe earthquakes. The country is well-endowed with spas and hot springs, which number 124 according to one North Korean source.