Which 297 km river is the longest flowing entirely on Greek soil?

The longest river flowing entirely within Greece, at 297 km, is the Haliacmon.

Also called Aliákmonas in modern Greek, the Haliacmon rises in the Pindus Mountains of western Macedonia and flows east and southeast through northern Greece before reaching the Thermaic Gulf. Its route includes the artificial Polyfytos Lake, created by a major hydroelectric dam.

Greece has three rivers longer than the Haliacmon overall: the Evros, Strymonas and Axios. Each begins in or crosses neighboring countries, however, so the Haliacmon is the longest river whose entire course lies on Greek territory. That geographic qualification is the key to the answer.

The river has also shaped ancient regional geography. Classical writers associated it with Macedonia and Thessaly, and ancient accounts describe changing connections between the Haliacmon and nearby waterways. Before modern flood-control and dam works, its lowland course was prone to flooding and marsh formation.