The Hers-Mort (the "Dead Hers", as opposed to the faster-flowing Hers-Vif, or "Live Hers") is a 89.3 km long river in southern France, a right-bank tributary of the Garonne. Its average flow rate is 4 m3/s. The Hers-Mort rises in the Lauragais region, near the village Fonters-du-Razès, in the Aude department. It flows northwest through the following departments and towns: It flows into the Garonne near Grenade-sur-Garonne. Its waters, augmented by the Girou which flows into its right bank, irrigate the market gardens around Toulouse. The Canal du Midi crosses the Hers-Mort near Villefranche-de-Lauragais via the Hers Aqueduct.

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