Map showing location of the Aliákmon

The Aliákmon (modern Greek: Αλιάκμονας, Aliákmonas; ancient Greek and Katharevousa: Ἁλιάκμων, Haliákmōn or Aliákmon; Latin: Haliacmon; Bulgarian and Serbian: Бистрица, Bistritsa/Bistrica; Turkish: İnce Karasu) is the longest river in Greece, with a total length of 322 km (200 miles). It is usually known as the Aliákmon in the English language, following the now obsolete Katharevousa form.

It rises in the northern Pindus mountains in northern Greece on the border with Albania, before flowing southeast then northeast through the Greek peripheries of West and Central Macedonia and then into Lake Kastoria, and into the dam and into the Thermaic Gulf. It waters an extensive area, making it of great importance to agriculture in the region. The river forms the western portion of the delta of the Axiós (aka Vardar). The river runs in the prefectures of Florina, Kastoria, Grevena, Kozani, Imathia, and Pieria.

Course of the Aliákmon river, as seen from the Terra satellite. (The river course has been highlighted.)

The Aliákmon flows along Kastoria, Neapoli, west of Siatista and into the Grevena prefecture and east of Grevena, and to the south of Kozani and into the Polyfytos dam which is 20 km long and about 4 to 5 km wide, and southeast of Aiane, and into the gorges and southeast of Veria and north of Aiginio and northeast of Methone.

In antiquity, the river was known as the Ἁλιάκμων (Haliákmōn), transliterated by the Romans into Latin as Haliacmon. The chain of mountains in which the river rises (northern Pindus) were named Canalovii by Ptolemy. According to Julius Caesar, the Haliacmon formed the line of demarcation between Macedon and Thessaly. In the upper part of its course it took a southeast direction through Elimiotis, which it watered; and then, continuing to the northeast, formed the boundary between Pieria, Eordaea, and Imathia. In the time of Herodotus the Haliacmon was apparently joined by the Loudias, or discharge of the lake of Pella; but a change has taken place in the course of the Loudias, which no longer joins the Aliakmon, but flows directly into the Aegean Sea.