The Vesgre is the main river crossing the Pays Houdanais. It arrives from the forest of Rambouillet, where it rises some 20 km to the southeast, near the Etangs de Hollande and flows into the Eure, north-west, at the Chaussee d'Ivry.
At the bottom of Houdan, more precisely in the grounds of the hospital, the Vesgre is joined by the Opton. The Vesgre as the Opton are capricious rivers which are able to flow out of bed easily. Their current is fast enough to power mills, private today.
This pond bypasses partially the farm of the Commandery. It was called "channels". A stream turned into a drainage ditch during the German occupation, feeds the pond.
Many farms were located in the village hence the importance of ponds. There are still a few ponds today:
This cross, erected in tribute to the first pilgrims gone from Houdan to Montserrat in 1582, is located on the edge of town on a hill accessible by road from the Abreuvoir tower. From here, a nice view over the city is offered to walkers.