Iran's two great rivers, the Tigris and the Euphrates, traverse Iraq from northwest to southeast, uniting as the Shatt al Arab water-way shortly before entering the Gulf. Dam-building in central Iraq has converted several large saline depressions into huge water-storage basins, the most important being Lakes Tharthar, Habbaniya and Rezzaza. In their lower courses, the Tigris and the Euphrates create a vast complex of shallow lakes and marshes, the Mesopotamian Marshes, home of the famous "Marsh Arabs".
Iran also possesses a great diversity of wetlands. In the north, there is an almost unbroken chain of freshwater lakes and marshes, brackish lagoons, irrigation ponds and rice paddies stretching along the southern edge of the Caspian Sea. Lake Orumiyeh is a vast hypersaline lake with spectacular breeding colonies of pelicans and flamingos. Although the lake is too saline to support any plants or animals other than the algae Enteromorpha sp. and the brine shrimp Artenia sp., the numerous small fresh and brackish water lakes and marshes along the rivers entering the lake support abundant aquatic vegetation and are very rich in wildlife.