By MIKE CATALINI and BRITTANY PETERSON Associated Press

TRENTON, N.J. (AP) — Salty oceanwater is creeping up the Delaware River, the source for much of the drinking water for Philadelphia and millions of others, brought on by drought conditions and sea level rise, and prompting officials to tap reservoirs to push the unpotable tide back downstream.

Officials say drinking water isn’t imminently at risk yet, but they’re monitoring the effects of the drought on the river and studying options for the future in case further droughts sap the area.

A closer look at the crawling salt front:

What is the salt front?

The salt front or salt line is where saltwater from the ocean and freshwater meet in the river. That boundary is typically somewhere around Wilmington, Delaware, but the recent drought in the Northeast has pushed it about 20 miles (32 kilometers) north, around Philadelphia International Airport.

Why does it matter?

The farther the line moves upstream, the closer it gets to drinking water intakes, which officials have worked for decades to avoid.

The Delaware River provides drinking water for some 14 million people, including most of Philadelphia but also New Jersey and New York. Still, the line is south of those intakes and below the level it traveled in the 1960s during record drought conditions.

Desalination of saltwater, which people cannot safely drink, is costly, energy intensive and can create new issues like where to dispose of the highly concentrated salt brine pulled from the water. It’s also not a feasible option, officials say.

“There are alternative sources, but we don’t want to be trucking in bottled water for people,” said Amy Shallcross, the water resource operations manager at the Delaware River Basin Commission. “We get nervous when it starts to get up near Philadelphia. It’s only 18 miles right now from the drinking water intakes. And sometimes it can shoot upstream really quickly.”

What are officials doing about the encroaching salt?

Officials control the salt line by releasing water from two reservoirs, which pushes the front downriver. The commission monitors the flow at Trenton, which is the furthest upstream point affected by the tide. The flow officials target is roughly equivalent to the amount of water in two Olympic-sized swimming pools flowing by per minute. If the rate dips below that, then more water is released.

When was the last time saltwater moved this far upriver?

The salt front last reached roughly where it is now in 2016 during another drought, officials said.