The longest river that originates in the country is the Aliakmon River, which flows through Western Macedonia and empties into the Thermaikos Gulf. The Haliacmon rises in the Gramos mountains in northern Greece, near the border with Albania. It feeds the large artificial Lake Polyfytos, that was created after the construction of the namesake hydroelectric dam and consists almost its entire course through the Kozani prefecture. It flows into the Thermaic Gulf west of the delta of the Axios (Vardar). Haliakmon's tributaries include Gramos, Ladopotamos, Pramoritsa, Grevenitikos, Venetikos and Tripotamos.