More than a century ago, New York City’s East River would freeze over every few decades, creating major issues for commuters who relied on ferries for access to Manhattan from the boroughs of Brooklyn and Queens. The problems were serious enough that they helped prompt the construction of the Brooklyn Bridge, which opened in 1883, as well as the city’s vast network of subways connecting the outer boroughs decades later.

Ice floes in the East River are far less common these days. It takes several days of sustained freezing temperatures for ice chunks to form, in part because the East River isn’t actually a river. I learned this a few years ago when I spoke to a company that was installing tidal turbines underwater just a few meters from my home on Roosevelt Island. The “river” is actually a 26-kilometer-long saltwater tidal strait that flows beneath the four major bridges—including the Brooklyn and Williamsburg crossings—that connect the borough of Manhattan to Brooklyn and Queens. Traffic on the river generally consists of tugboats moving barges either north toward the Long Island Sound or south to Upper New York Bay.

The East River runs in either direction, depending upon the time of day, and its waters can move in excess of five knots—forceful enough to have destroyed the original set of tidal turbines installed in 2002. Redesigned turbines fared better. As a result, in the many years that I’ve lived on my small sliver of land in the East River, I’ve never seen ice form on its surface.

Until this year.