The nature of the Graeco-Roman bridge at Zeugma is uncertain. Nothing now remains of the bridge and nothing was discovered in the excavations and survey of the last 15 years.
We are left, therefore, with the written evidence, often by people who had never seen the bridge and were reporting what they read elsewhere (also, often, by people who had never seen it) or what popular belief "told" them about this famous bridge. The likeliest interpretation of the evidence in the ancient written sources (below) is that the bridge was in fact a pontoon bridge or, perhaps later, a mix of fixed and pontoon.
The nature of Roman pontoon bridges can be seen in a scene on Trajan's Column in Rome.
Permanent stone bridges were common enough. One is illustrated on Trajan's Column again – including the Suovetaurilia sacrifice mentioned by Plutarch.
There are also superb examples surviving. One of the best is at Merida in Spain.
My best interpretation of the evidence and of what would be needed at Zeugma is a mix of the two types. These mixed types were still in use on the Tigris at least in modern times. Their advantage is that they combine the solidity and permanence of a stone bridge extending across most of the river with the flexibility of a short pontoon section that could be unhitched in bad weather or to allow ships to pass through. Examples can be seen in an early 19th century drawing and in two old RAF aerial photographs of Mosul on the Tigris in modern Iraq in 1934 (below).