Shornhelm's substructure includes many interconnected cisterns, a honeycomb of pocketed caches which is connected to every potable well within the city.
Many of them leak into each other as a consequence of age and shoddy craftsmanship, but only one distributes water on a grander scale! This cistern is referred to as the master source, as it allows flow from an underground river directly into every smaller cistern in the city.
To open the distribution channels you must first open both sluice gates, then turn the winch to start the external water flow.
To clear the cistern before distribution of any residual water, close both sluice gates and then turn the winch. This will flush all water from the system in a matter of days!