Tigris–Euphrates river system through the central plain and combine at Al-Qurnah to form the Shatt al-Arab and discharge into the Persian Gulf. The region has historical importance as part of the Fertile Crescent region, in which civilization is believed to have first emerged. The ecoregion is characterized by two large rivers, the Tigris and Euphrates. The rivers have several small tributaries which feed into the system from shallow freshwater lakes, swamps, and marshes, all surrounded by desert. The hydrology of these vast marshes is extremely important to the ecology of the entire upper Persian Gulf. Historically, the area is known as Mesopotamia. As
Chicago River lowest since 1918. In 2012 Lake Michigan-Huron's seasonal rise was about 4 inches where it usually is about 12 inches. Normally the Chicago River water level is two feet lower than the lake and therefore does not flow into the lake. If the lake level falls too low threatening to reverse the river flow, the Metropolitan Water Reclamation District of Greater Chicago would be forced to close locks between the lake and river for longer periods of time limiting navigation. A reversal flow of the Chicago River into Lake Michigan would have a negative impact on navigation and on the