The nation's drainage system is essentially landlocked. Most of the rivers and streams end in shallow desert lakes or oases inside or outside the country's boundaries. Nearly half of the nation's total area is drained by watercourses south of the Hindu Kush-Safid ridge line, and half of this area is drained by the Helmand and its tributaries alone. The Amu Darya on the northern border, the country's other major river, has the next largest drainage area.