Residents of Trenton and surrounding towns are being urged to conserve water as icy conditions on the Delaware River impede the drinking water supply.

Trenton Water Works — the utility that provides drinking water to more than 200,000 people across Trenton and parts of Ewing Township, Hamilton Township, Hopewell Township and Lawrence Township — put out an advisory Wednesday night asking customers to use water only for essential reasons or emergencies.

The problem is attributed to frazil ice buildup in the Delaware River, the system’s main raw water source. Frazil ice is a collection of small ice crystals that form in water while it’s still moving when temperatures drop below freezing. It’s notorious for blocking water intakes, and effectively shut down operations at the Trenton water filtration plant.

A spokesperson for the city says there’s still plenty of safe drinking water while the water filtration plant is offline. Trenton Water Works is relying on its Pennington Avenue Reservoir, which is designed to store 100 million gallons of treated water. That’s about a three-day supply for the system.

Trenton Water Works has also activated an emergency interconnection with the nearby New Jersey American Water system while the city works to get the water filtration plant operating.