[3] The Thouet rises in the Gâtine Vendéenne, the most southern outcrop of the Armorican Massif, at 225 metres (738 ft) altitude.

Between Saint-Loup-Lamairé and Thouars, the river flows onto the Paris Basin, an area of sedimentary rocks where successive marine deposits of periods from the Triassic to the Pliocene were laid down.

The nature of the rocks here allows the formation of an aquifer whose water reserves support the river in times of low flow.

As part of this development, two modern pound locks with straight sides and mitred gates were built on the Thouet, downstream of the confluence with the Dive.

The maximum boat size imposed by the locks proved insufficient, and a proposed enlargement to the larger Freycinet gauge was never realized.