Caspian Sea basin

The rivers flow to Caspian Sea in this basin and their total watershed is 173,300 Km² this basin has steep dips and is the highest basin (5500m) in the country. Therefore Aras, Haraz, Sefidrood and Atrak are wide basins with steep dips and short length.

Annual atmospheric precipitation is rain in this basin and snow melting does not affect their water-supply. Daily variation of water-supply in these rivers is very intensive but it does not change much annually.

Aras, Sefidrood, Haraz and Atrak are different from other rivers in this basin climatologically. Their water-supply is provided by snow melt.

There are 13 rivers which basins are more than 1000 Km² in Caspian Sea basin where Aras and Sefidrood are the most important ones.

Watershed forms 3 main regions in this basin:

A-   Rivers with wide mountainous water-supply.

B-   Rivers that part of their watershed basin is high mountainous area and another part of their basin are East seafront low amplitude areas. This group has a more regular seasonal distribution in comparison to group A. this group is influenced by permanent precipitation of mountainous areas. Gorgan, Tajen, Talar, Polrood and Shafarood are in this group.

C-  Rivers that their watershed basin is forest areas and their water-supply is rain precipitation. These rivers have a torrent current and very low permanent water flow. The small rivers are in this group.