I was at = I was in. Daniel may have retired there (during the lycanthropy of Nebuchadnezzar) when Nehemiah and Mordecai were in the court of Astyages (Neh_1:1). That Daniel was there employed by Astyages is clear from Dan_8:27.
Shushan. The chief city of all Persia.
river. Hebrew. 'ubal = canal. Only here, and in verses: Dan_8:3, Dan_8:6.