<P> There are no major rivers in the country . Of the small rivers and streams, the only one that is navigable is the 830 kilometres (520 mi) - long Karun, which shallow - draft boats can negotiate from Khorramshahr to Ahvaz, a distance of about 180 kilometres (110 mi). Other major rivers include the Karkheh, spanning 700 kilometres (430 mi) and joining the Tigris; and the Zayandeh River, which is 300 kilometres (190 mi) long . Several other permanent rivers and streams also drain into the Persian Gulf, while a number of small rivers that originate in the northwestern Zagros or Alborz drain into the Caspian Sea . </P> <P> On the Central Plateau, numerous rivers--most of which have dry beds for the greater part of the year--form from snow melting in the mountains during the spring and flow through permanent channels, draining eventually into salt lakes that also tend to dry up during the summer months . There is a permanent salt lake, Lake Urmia (the traditional name, also cited as Lake Urmiyeh, to which it has reverted after being called Lake Rezaiyeh under Mohammad Reza Shah), in the northwest, whose brine content is too high to support fish or most other forms of aquatic life . There are also several connected salt lakes along the Iran - Afghanistan border in the province of Baluchestan va Sistan . </P> <P> Iran has a variable climate . In the northwest, winters are cold with heavy snowfall and subfreezing temperatures during December and January . Spring and fall are relatively mild, while summers are dry and hot . In the south, winters are mild and the summers are very hot, having average daily temperatures in July exceeding 38 ° C (100.4 ° F). On the Khuzestan Plain, summer heat is accompanied by high humidity . </P> <P> In general, Iran has an arid climate in which most of the relatively scant annual precipitation falls from October through April . In most of the country, yearly precipitation averages 250 millimetres (9.8 in) or less . The major exceptions are the higher mountain valleys of the Zagros and the Caspian coastal plain, where precipitation averages at least 500 millimetres (19.7 in) annually . In the western part of the Caspian, rainfall exceeds 1,000 millimetres (39.4 in) annually and is distributed relatively evenly throughout the year . This contrasts with some basins of the Central Plateau that receive ten centimeters or less of precipitation . </P>

How does iran's geography affect its economy