Much of the Kabul's course is tapped for irrigation, so much so that west of Kabul city the river often dries up in summer. Irrigation is also extensive in the Jalalabad and Peshawar areas. A few miles below the junction with the Panjsher, a hydroelectric plant has been built. The Kabul River valley is a natural route for travel between Afghanistan and Pakistan; the Macedonian Alexander the Great used it to invade India in the 4th century BC. Since 1945 the Peshawar-Jalalabad-Kabul Highway has occupied the valley. The river is navigable by flat-bottomed vessels below Kabul city.