The Sistan region has varied in name and dimensions over the millennia but largely refers to the closed area at the southwest corner of the larger Helmand River basin, west of the towering volcano Koh-i Khan Neshin to the hamun (lake) into which it drains, located on the border between Afghanistan and Iran. This 18,000 sq km landlocked depression is noted for its dry climate, extreme temperatures, and strong winds. From its narrow course 2-5 km wide near the mountains, the river widens into a delta-like basin and disperses its flow into several distributary channels and canals. Bordered by flat Dasht-i Margo and Dasht-i Jehannum deserts on the east, Dasht-i Lut desert on the west, and the sand-covered Registan to the southeast, it is one of the few habitable areas in southwest Afghanistan and southeast Iran because of its perennial flow of water.
Geological work by John Whitney and colleagues suggests the course of the Helmand River through Sistan was affected by tectonic factors, including a major fault that deflected the river northward, and by pronounced subsidence of the basin as well as surface factors such as the annual flooding in the spring and the strong winds that erode the landscape.