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
Castlereagh River travel the distance, and up to 47 hours. Usually in peak floods, the greater the water volume, the faster the travel times of the peak. However, the flattening of the terrain upstream of Coonamble results in decreased flow velocity (but means a greater area for the flow is needed to avoid flooding). It is not correct that the Castlereagh River is an underground river (subterranean river) or an 'upside down' river. However, water that once was in the river bed on the surface does flow through the extensive sands under the riverbed. This water is therefore part of the underground