When summer heat dried up the river, a specialized architectural team was able to conduct excavation on one bank, discovering the rectangular bases of large pillars that would have supported the bridge, the Italian news agency ANSA reported.
Their large size, 30 feet by 15 feet, is evidence of a bridge with considerable width, researchers said.
Previous archaeological work in the area from 1963 to 2003 also suggested the bridge was probably about 650 feet wide and was supported by 11 pillars.
Additional work will be carried out under the scientific supervision of the Italian Culture Ministry to discover if the Roman bridge was based on natural foundations or man-made ones, researchers said.
The Isonzo River flows through both Italy and Slovenia where it is known as the Soca River.