Three rivers flow through the territory of Grandlieu. Boulogne river About 90 km long, the river begins in Saint Martin des Noyers, Vendée, runs to the north and into Grandlieu lake. Among others, it flows across Saint Colomban and Saint Philbert de Grand Lieu. Rivière L'Ognon Ognon river This 48,4 km long river begins in Saint Sulpice le Verdon, Vendée. It runs to the south-east and north-west before joining Grandlieu lake. It flows across several towns including Montbert, Pont Saint Martin and La Chevrolière. Rivière La Logne Logne river The Logne river begins in Les Lucs sur Boulogne, Vendée. 34 km further, it joins the Boulogne river in Saint Philbert de Grandlieu. Among others, it flows across La Limouzinière and Saint Colomban.