Iranian Rivers Major mountain ranges and their topography have an effective impact on precipitation regimes and surface water formation. Alborz and Zagros, the most important mountain ranges, are determining factors of precipitation in the country. Snowing in the height is water supply of rivers. The most important factor of precipitations is the air current that moves from the west and Mediterranean basin towards Iranian platue and loses its moisture in Alborz and Zagros, resulting snow and rain in the autumn and winter. Part of the precipitations penetrates in to Zagros limestone and forms many springs. Precipitation in the spring season causes strong floods and supplying part of the surface water.