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, p 6
Like the Tigris, the Euphrates begins in the mountains of Armenia (Jastrow 1915, pg. 5). It flows westerly toward the Mediterranean, fed by many tributaries; then the Euphrates suddenly veers southeast and is fed by few tributaries until joining the Tigris in the extreme south at Kurna to form the Shatt el-Arab (aka Arabaic River) (Jastrow 1915, pg. 5-6). Its 1780 miles are only briefly navigable: in the north, it is hindered by cataracts; in the south, its sluggishness leads to sand banks. Thus, the Euphrates was never a critical commercial avenue. The only transportation along the Euphrates was via rafts and wicker baskets coated with and without bitumen.
There are many archaeological sites.
Mari has provided more than 20,000 texts written in Old Babylonian, found in destroys remains of the palace of Zimri-lin (~ BCE) and regarding administration and royal correspondence. Primary source for identifying \"city states\" as actual kingdoms are the inscriptions found at Mari.
Hasanlu (aka Mania) was a well-preserved city caught between the Urartians and Assyrians, and others. It had pseudo-Assyrian art styles, and was eventually obliterated in an utterly destructive attack likely by the Babylonians (due to the preponderance of scythian arrowheads).