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The Haliacmon is the longest river in Greece, with a total length of 322 km (200 miles). Haliacmon is the traditional English name for the river, but many sources cite the formerly official katharevousa version of the name, Aliákmon. Today, the only official variant is the demotic Aliákmonas.

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 Kastoria, Grevena, Kozani, Imathia, and Pieria.

In antiquity, Claudius Ptolemy called the chain of mountains in which the river rises (northern Pindus) the Canalovii. 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 Haliacmon, but flows directly into the Aegean Sea. The image below shows a wind gap between the Haliacmon and Loudias watersheds that is the probable ancient course of the Haliacmon.