Time-lapse of Hudson River ice – a glimpse of the Hudson ‘breathing’

In this time-lapse video by Michael Levine, you see the transition from flood to ebb along the Hudson River at Hastings.

The change of the tide occurs approximately every six hours. Only when there’s ice, and only thanks to this time-lapse photographer, do we get a glimpse of the Hudson breathing.

The dynamic mixing caused by the tides is one of the reasons that estuaries like the Hudson are so rich in life. Nutrients are held up in the water column for aquatic life to use.

In addition to what you can see of the tidal movement, one of the most fascinating aspects of the estuary is the mixing of salty ocean water and fresh riverine water.

Typically the salt front is around Stony Point, where the estuary becomes effectively “fresh” – though during drought, the salt front has moved as far north as Poughkeepsie. But the transition from salt to fresh is not a wall, as “front” would imply. In reality there’s a constant battle between heavy, dense salt water from the ocean trying to fill estuary and lighter, lower density, fresh water from the watershed trying to push the ocean back.

The result is a “salt wedge” – invisible to us mere mortals who walk the land, but critically important to aquatic life in the lower estuary.

So at any given time at the George Washington Bridge, or off Yonkers, or off Tarrytown, the deeper water is saltier than the upper layers. And every day, the mix is different, depending on the power of the tides – a function of the lunar cycle, time of day, weather systems, atmospheric pressure, and rainfall in the Hudson River watershed.

There are even times when the current in the lower, deeper half of the river is moving in one direction while the surface is moving in the opposite direction. The water movement at the surface and at depth in the Hudson estuary is invisible to us – but is its very life.