The site of the battle was at the city of Opis on the river Tigris, located about 50 miles (80 km) north of modern Baghdad. The city is thought to have been a preferred point to cross the river; Xenophon describes a bridge there. The timing of the invasion may have been determined by the ebb of the Mesopotamian rivers, which are at their lowest levels - and therefore are easiest to cross - in the early autumn.