The river whose sediment has greatly changed the landscape around Thermopylae is the Spercheios. Its steady deposits have transformed a narrow coastal pass into a broad floodplain and pushed the shoreline farther into the Malian Gulf.
In 480 BCE, Thermopylae was much narrower than it is today, with the sea lying close to the mountain slopes. Sediment carried downstream by the Spercheios, combined with deposits from the area's hot springs, gradually raised and extended the land.
The ancient battlefield is now buried beneath roughly 20 metres of soil. This dramatic change is why modern visitors do not see the same narrow coastal corridor described in accounts of Leonidas and the Persian invasion.
The other options are rivers associated with different Greek regions. The Cephissus, for example, is linked primarily with central Greece and Boeotia, while the Eurotas flows through the Spartan homeland in the Peloponnese.