Most ancient dams were simple affairs, an earthen rampart or low wall, constructed to act like a weir and control water flows rather than to create reservoirs. The latter occurs generally only in the drier regions of the Roman Empire. Nero's dams across the river Anio at Subiaco in Italy were an exception; these were to create three pleasure lakes in the grounds of his palace there. Trajan, typically, tapped these lakes to feed the Anio Novus aqueduct, and thus made them useful, but they were not built to feed aqueducts. The largest dam here was about the same height as the Pont du Gard; there is apparently a painting of it in the monastery of Sacro Speco nearby. The dam survived until a monk stripped the protective top tiles from it around 1305 AD...
It would appear from recent work at Nemea that earthworks for flood control, and possible even landscape management, go back at least to the C6 BC. The excavators of the site note that the long N-S artificial mound near the shrine of Opheltes serves as a dike, and believe that it might have been intended as such, protecting the core of the sanctuary to its east from flooding, and creating a flood plain suitable for a hippodrome to its west. Archaeological Reports for 2003-4 (2004) 19.
Caesar made wood and earth dams wherever necessary at Dyrrachium in order to cut off all the streams and rivulets that were flowing towards Pompey's forces.
The most remarkable known dam in antiquity is the Daras dam. It was an arched dam, that is to say, it was built as we build dams, a relatively thin structure shaped like an arch laid on its side, with the curve laying towards the body of water and thus working like an arch to transfer to weight of the water to the rock on either side of the valley.
The dam was built c. 550 AD, not to create a reservoir, but as a measure in flood control for a city that had previously been seriously damaged by a river in flood. This had included the city gate being burst open by the sudden floodwaters. Sluice gates were built into the dam at lower and upper levels, so that when the river rose in flood the dam would contain the surge and only an 'orderly' quantity of water would be allowed to flow on into the city of Daras. Daras was on the eastern edge of the Roman empire, bordering Persia. It was built by Anastasius and refurbished by Justinian.
It seems to me possible that something similar was built in the upper reaches of the Sperkhios valley, this time primarily with a view to defense. Justinian closed the narrow pass with a very strong cross-wall 'which he made fast to the mountains either side'. This stopped the barbarians entering by that route and forced the stream in flood to pond behind it and then flow over it (Procopius 4. 2. 19-22). Since we have earlier been told that this stream forms 'an exceedingly deep and very violent torrent whenever it rains', and the ravine appears to be suitable for an arched dam, one might have been built here.
Reference: for Daras dam see Procopius Buildings 2. 3. 16-20. Caesar Civil Wars 3. 49.