Like the Euphrates, the Tigris begins in the mountains of Armenia and (Jastrow 1915, pg. 5). The Tigris quickly gains strength upon leaving its source, forcing its way through rugged clefts and getting fed by numerous tributaries before even reaching the plain (Jastrow 1915, pg. 6). Along its brief 1146 miles, it is navigable from Daiarbekr in the north until it unites in Kurna with the Euphrates, thus forming the Shatt el-Arab (aka Arabic River).
Large rafts can be floated down to Baghdad and small streamers can ascend almost to Mosul [ancient Nineveh]. The Tigris is, therefore, the avenue of commerce for Mesopotamia … and forms the link that connects Babylonia and Assyria through the Persian Gulf with India on the one hand, and Egypt and the Red Sea and the Mediterranean districts on the other. (Jastrow 1915, pg. 6)