Almost 80 percent of the territory of Turkmenistan lacks a constant source of surface water flow. Its main rivers are located only in the southern and eastern peripheries; a few smaller rivers on the northern slopes of the Kopetdag are diverted entirely to irrigation. The most important river is the Amu Darya, which has a total length of 2,540 kilometers from its farthest tributary, making it the longest river in Central Asia. The Amu Darya flows across northeastern Turkmenistan, thence eastward to form the southern borders of Uzbekistan and Tajikistan. Damming and irrigation uses of the Amu Darya have had severe environmental effects on the Aral Sea, into which the river flows (see Environmental Issues, this ch.). The river's average annual flow is 1,940 cubic meters per second. Other major rivers are the Tejen (1,124 kilometers); the Murgap (852 kilometers); and the Atrek (660 kilometers).