The terrain is rugged and mountainous, especially in the north and in the Annam Range, along the border with Vietnam. The mountains reach heights of more than 2,700 m (8,860 ft), with Pou Bia, the highest point in Laos, rising to 2,817 m (9,242 ft) in the north-central part of the country. Only three passes cross the mountains to link Laos with Vietnam. The Tran Ninh Plateau, in the northeast, rises to between 1,020 m and 1,370 m (3,350– 4,500 ft), and the fertile Bolovens Plateau, in the south, reaches a height of about 1,070 m (3,500 ft). Broad alluvial plains, where much of the rice crop is grown, are found only in the south and west along the Mekong River and its tributaries. Of these, the Vientiane plain is the most extensive.
Except for a relatively small area east of the main divide, Laos is drained by the Mekong and its tributaries. The Mekong flows in a broad valley along the border with Thailand and through Laos for 1,805 km (1,122 mi). In its low-water phase, it is almost dry, but it rises more than 6 m (20 ft) during the monsoon period. The river is wide, but except for a navigable stretch between Vientiane and Savannakhét, rapids are numerous. Below Savannakhét and at the extreme south there are large rapids and waterfalls. Floods are common in the rainy season.