The Great Zab rises in the highlands of the Zagros Mountains, where a climate with cold winter and annual precipitation in excess of prevails. From there, the river flows into the foothill zone of the Zagros, where rainfall drops to less than per year at the confluence with the Tigris. Average summer temperature in the foothill zone are generally higher in the foothill zone than in the mountains. The high Zagros is characterized by three different ecozones: the area above the treeline at where shrubs and herbs dominate, the area between that was in the past dominated by open oak forest (Quercus aegilops), and the wetter and sometimes marshy river valleys. Other trees besides oak that can be found in the forested zone including juniper at higher elevations; ash, hawthorn, maple and walnut at intermediate elevations; and pistachio and olive trees in lower, drier areas. In the foothill zone, many areas are now cultivated, but there remain small patches of natural vegetation dominated by herbs of the genus Phlomis.