The currents through the Narrows yesterday were about as strong as they get: a 4.8 knot ebb in the morning and a 5.6 flood later in the day. For a waterway that's a mile wide at its narrowest point, that's some serious velocity. I am not going to pretend that I can do the math, but I know that the amount of moving water represented by those kinds of current speeds is immense.
It wasn't hard to see either. The surface of the water was moving in sheets and boils and upwellings were visible all over. As the tide fell, the beach grew wider at a pace that was easily discernible, and the same was true in the afternoon as well.
It's because of the moon, as is normally the case when it comes to matters like these. Yesterday's full moon was a supermoon, meaning that it occurred at a point when the moon was actually about 15,000 miles closer to the Earth than is typically the case and pushed the water around the planet even more than usual. The power of the moon should not be underestimated.
The proximity of last night's moon made it look even bigger than a normal full moon looks too. I happened to be up around 4:30am, when the moon was nearing our western horizon, and it looked bigger, yellower and closer. (I just looked outside a few minutes ago and the waning gibbous we've got tonight, while still beautiful, doesn't have the same magic.)