The Helmand drains about 40% of Afghanistan, and the Hari Rud runs west into Turkmenistan

Background

The Food and Agriculture Organization's technical paper on fish and fisheries at higher altitudes records that nearly 75% of Afghanistan is mountainous. The average elevation is 1300 m, and the northeast is sub-polar, with temperatures falling to -26 degrees Celsius or lower in the Hindu Kush range.
The paper traces the water in every Afghan river to precipitation and the snow melt that follows, over the central ranges running from the Pamir mountain knot at the western termination of the Karakoram, through the Hindu Kush and its outliers, to the ranges of Hazarajat.
The Amu Darya has its sources in the Pamirs, and it ends in the Aral Sea; the lower 1300 km of a total of 2500 km lies wholly outside Afghanistan.
The Hari Rud River, which starts in the centre of Afghanistan, flows directly west and eventually enters Turkmenistan where it is called Tedzhen, to be lost in the sands of the Kara Kum Desert.
The Helmand River has its source not far away from the source of the Kabul River. It flows southwest for about 1300 km before it empties into the Sistan lakes, and with its tributaries it drains about 40% of Afghanistan and has the largest drainage basin.
Because the supply is snow melt, maximum water flow is in the spring and early summer and minimum flow is in late summer to winter over much of the country.