Why was the Tigris not dried up, too?

Revelation 16:12 And the sixth angel poured out his vial upon the great river Euphrates; and the water thereof was dried up, that the way of the kings of the east might be prepared.

During the Iran/Iraq war, I was working, for a while, in a refinery near the Tigris river.

I was told that at the beginning of the war, that Iranian planes came suddenly over the Tigris to bomb the refinery.

So, apparently the kings of the east won't be coming that way, from Iran?

The Euphrates starts in Turkey, and comes through Syria, before going down through Iraq.

So is it possible that the portion in Turkey and/or Syria must be dried up to prepare the way of the kings of the east, coming down that way?