We meteorologists talk a lot about fronts - for example, warm and cold fronts are always prominent features on weather maps. Around here, there's another front that's not in the news much but is nonetheless nearby every day - it's called the salt front, also known as the "salt line," and you can find it in the Delaware River.

The Delaware is tidal up to about Trenton, so salt-laced water from Delaware Bay does infiltrate the lower reaches of the river. The front edge of that relatively salty water is the salt front. Its location is determined using drinking water standards. This time of year, its average location is near the Delaware-Pennsylvania state border, about ten miles north of the Delaware Memorial Bridge. That's about where the salt line is today. But it does advance and retreat, controlled primarily by the downstream flow rate of the river. That, in turn, depends very much on how much rain has fallen in the Delaware River basin.

During droughts, the flow rate of the river is reduced, so the salt front advances upriver. It was around this time in 1965, when we were in the throes of the worst drought on record in this area, that the salt front made its farthest upstream advance. It crept all the way north to just past the Benjamin Franklin Bridge. Had the salt line moved another ten miles upstream, it would have reached a water intake plant for Philadelphia, contaminating half the city's water supply.