The main source of the Tanarello is on the slopes of Monte Saccarello above Monesi, a village belonging to the commune of Triora. This mountain straddles the French département of Alpes-Maritimes, the Piedmontese province of Cuneo and the Ligurian province of Imperia and marks the juncture of the watersheds between three drainage basins: that of the Tanaro itself; that of the Roya (Italian Roia), which rises in France but enters the sea at Ventimiglia; and the Argentina, which flows into the Ligurian Sea at Taggia.