Jamieson, Fausset and Brown on Ezra 8:15

Ezr 8:15-20. He Sends to Iddo for Ministers for the Temple Service.

15. I gathered them together to the river that runneth to Ahava--This river has not been ascertained. It is probable that the Ahava was one of the streams or numerous canals of Mesopotamia communicating with the Euphrates [Cyclopædia of Biblical Literature]. But it was certainly in Babylonia on the banks of that stream; and perhaps the place appointed for general rendezvous was in the neighborhood of a town of the same name. The emigrants encamped there for three days, according to Oriental custom, while the preparations for the departure were being completed and Ezra was arranging the order of the caravan.

I ... found there none of the sons of Levi--that is, the ordinary Levites. Notwithstanding the privilege of exemption from all taxes granted to persons engaged in the temple service, none of the Levitical tribes were induced to join the settlement in Jerusalem; and it was even not without difficulty Ezra persuaded some of the priestly families to accompany him.