In the books of eschatology it is placed as a sign by preceding Imam Mahdi. That is stated by Hafiz Ibn Hajar Makki, Imam Suyuti, Imam Barzanji and others. The hadith mentioning the drying of the Euphrates as recorded by Nuaym bin Hammad in Fitan mentions this as a sign preceding the appearance of Imam Mahdi. The hadith of the army being swallowed in the Arabian desert will take place before the appearance of Imam Mahdi. It is important that this issue be highlighted as many have deviated because of Mahdi claimants, or sufis who claim to represent the Mahdi on earth and claim knowledge which no one else has, all the while they contradict the authentic hadith. Clear example of this, for instance, is Ahmad Hasan from Basrah, who formerly was a Twelver: http://wup-forum.com/viewtopic.php?f=113&t=24470 Or Harun Yahya who believes the Mahdi will be a Turk: http://whoisharunyahya.wordpress.com/harun-yahya-or-adnan-oktar-the-promised-mahdi/ This also takes the form of sufi cults that claim a mystical knowledge of the Imam Mahdi's whereabouts or activity, as well as some of them alluding to their particular shaykh possibly being the Imam Mahdi!! Here some people entertain that Shaykh Nazim Haqqani's son may be the Mahdi: http://berabbani.com/mahditime.jpg http://berabbani.com/MAHDIHIDDEN.jpg http://berabbani.com/ Or Habib Ali Jifri as Imam Mahdi: http://www.sunniport.com/masabih/showthread.php?t=8547
The worst hit are the twelve shia areas of southern Iraq. i believe that the Euphrates as dried up by 50% there. This is the reason why there is a continuous stream of twelvers migrating to Baghdad in search of non-agricultural life, filling in empty homes of Sunnis. All weight is now on the Tigris, and diverting water from it. So, its not all hell loose for the twelvers in southern Iraq. The Iraqi government has big projects of development for southern Iraq, which is the most progressing part of Iraq in terms of infrastructure.
Imam Mahdi cannot appear until the Euphrates River dries up, including a host of other signs (like an army being swallowed in the Arabian Peninsula): http://www.youtube.com/watch?v=uJbMO2Zg2C0
