The central highlands—actually a part of the Himalayan chain—include the main Hindu Kush range. Its area of about 160,000 square miles (414,000 square km) is a region of deep, narrow valleys and lofty mountains, some peaks of which rise above 21,000 feet (6,400 metres). High mountain passes, generally situated between 12,000 and 15,000 feet (3,600 to 4,600 metres) above sea level, are of great strategic importance and include the Shebar Pass, located northwest of Kabul where the Bābā Mountains branch out from the Hindu Kush, and the storied Khyber Pass, which leads to the Indian subcontinent, on the Pakistan border southeast of Kabul. The Badakhshān area in the northeastern part of the central highlands is the location of the epicentres for many of the 50 or so earthquakes that occur in the country each year.
The northern plains region, north of the central highlands, extends eastward from the Iranian border to the foothills of the Pamirs, near the border with Tajikistan. It comprises some 40,000 square miles (103,000 square km) of plains and fertile foothills sloping gently toward the Amu Darya (the ancient Oxus River). This area is a part of the much larger Central Asian Steppe, from which it is separated by the Amu Darya. The average elevation is about 2,000 feet (600 metres). The northern plains region is intensively cultivated and densely populated. In addition to fertile soils, the region possesses rich mineral resources, particularly deposits of natural gas.
The southwestern plateau, south of the central highlands, is a region of high plateaus, sandy deserts, and semideserts. The average elevation is about 3,000 feet (900 metres). The southwestern plateau covers about 50,000 square miles (130,000 square km), one-fourth of which forms the sandy Rīgestān region. The smaller Mārgow Desert of salt flats and desolate steppe lies west of Rīgestān. Several large rivers cross the southwestern plateau; among them are the Helmand River and its major tributary, the Arghandāb.
Most of Afghanistan lies between 2,000 and 10,000 feet (600 and 3,000 metres) in elevation. Along the Amu Darya in the north and the delta of the Helmand River in the southwest, the elevation is about 2,000 feet (600 metres). The Sīstān depression of the southwestern plateau is roughly 1,500 to 1,700 feet (450 to 500 metres) in elevation.
Practically the entire drainage system of Afghanistan is enclosed within the country. Only the rivers in the east, which drain an area of 32,000 square miles (83,000 square km), reach the sea. The Kābul River, the major eastern stream, flows into the Indus River in Pakistan, which empties into the Arabian Sea of the Indian Ocean. Almost all the other important rivers of the country originate in the central highlands region and empty into inland lakes or dry up in sandy deserts. The major drainage systems are those of the Amu Darya, Helmand, Kābul, and Harīrūd.
The country possesses extremes in the quality of its soils. The central highlands have desert-steppe or meadow-steppe types of soil. The northern plains have extremely rich, fertile, loesslike soils, while the southwestern plateau has infertile desert soils except along the rivers, where alluvial deposits can be found.