New York Harbor is the confluence of multiple waterbodies with the Upper Bay subject to strong tidal currents that can reach over 6 kts on the East River. There are multiple factors that come into play: the south flowing current of the Hudson River, the tidal influence of the Atlantic through the Narrows and Long Island Sound, and the many other smaller tributaries that flow out into the Bay and surrounding waters.

New York City experiences a semi-diurnal tidal cycle, meaning that the tide will change from High to Low roughly twice in a 24 hour period. When navigating, it is crucial to understand the difference between tide and current. While both are important factors for boaters – and related – they each refer to different measurements. High tide (and conversely, Low tide) is the time when the water level is at its highest (or lowest). Current refers to the speed at which the surface of the water is moving.

As the images above illustrate, the flow of current around Manhattan changes with tides: the current is in Flood (flowing north on the Hudson and East Rivers) before and after High tide, the current is in Ebb (flowing south on the Hudson East Rivers) before and after Low tide. The time between north and south flow is considered Slack, when the current is at its weakest as it changes direction. A helpful shorthand to understand this cycle is the ‘Rule of Thirds‘ which describes roughly the changing speed of current throughout the tidal cycle. It can be remembered by 1:2:3:3:2:1, where for each hour between slack before high and slack before low, the current will increase by 1/3 its maximum rate. So 1/3 of max current in the first hour, 2/3 of max current in the second hour, max current (3/3) in the third and fourth hour, as so on.