Overall, about two-thirds of the country receives less than 250 mm of rainfall per year. The rainfall characteristics of the above basins are summarized in Table 4. All these basins, except the Persian Gulf and Gulf of Oman, are interior basins. There are several large rivers, the only navigable one of which is Karun, the others being too steep and irregular. The few streams that empty into the Central Plateau dissipate into the saline marshes.
All streams are seasonable and variable.
Spring floods do enormous damage, while there is little water flow in summer when most streams disappear. It can also be tapped by wells. Dams have always played an important role in harnessing Iran's precious water reserves and the long-term objective of Iran's water resources development plan is based on the control and regulation of water resources through dams. Dams also play an important role in flood control through routing of floods. Several reservoirs behind the dams seem to offer good sailing and water-skiing facilities, but have not been used for recreation so far.