The Block Island Times

How do the tides make it into the Great Salt Pond?

By Susan Willis | Nov 15, 2013

Imagine holding one side of a rope while a friend holds the other. When you move your hand up and down, you create a wave that travels down the rope to your friend. Water waves are similar: waves are energy moving through the water, like the wave moving on the rope!

Tides are the result of waves moving at a really, really low frequency (that is, how often a wave crest occurs). Tides are connected to the moon’s location relative to the earth. High tides are created on each side of the earth. One happens when the moon is near the water (pulled by gravity). The other occurs when the moon is all the way on the other side of the earth and the water doesn’t feel the pull of the moon; water rushes away, creating the other high tide. The effect of the sun’s gravity does a similar thing, only on an even lower frequency, creating spring and neap tides.

If we can think about tides as really slow waves, we can imagine the height of the water surface moving up and then down very slowly. But where does the water come from when the tide comes in to the Great Salt Pond?

In semi-enclosed basins, such as the GSP, the way water enters and leaves the basin is complicated. If you’ve ever stood at the cut at the old Coast Guard station, you’ve probably seen water rushing by (along with various boats, and maybe your bait being carried off by a fish). If you’ve stood in it, you can feel it, too. That water has to move very fast, up to several feet per second, to try to fill and drain the Great Salt Pond to match the tides outside the cut. As the tide rises (the tidal wave approaches its crest), the water level rises in the ocean outside the Great Salt Pond. This is a buildup of water with a bunch of extra pressure — which it releases by shooting water through the cut into the pond.

A similar thing happens on a falling tide. The water level outside the pond falls with the tide, but the level inside the pond is higher — so to release its pressure, water flows back out the cut and into the ocean.

Interestingly, the high tide water level in the pond won’t reach the same level (relative to the earth’s surface) as that in the open ocean, because there is not enough time to fill the basin of the pond. The pond will continue to fill even as the tide outside has started to ebb, since the water level outside is still higher. It also takes a little while for the water to spread out in the pond. These two things together explain why there is a lag between high tide at the North Light and high tide at Payne’s Dock: it simply takes longer to get to its maximum level.

On a larger scale, this same principle explains why high tide in Providence lags behind the high tide in Newport. Narragansett Bay acts like the cut in the Great Salt Pond, slowing down the tidal wave and dampening its maximum height.

Try these!

1) Search the internet for “Bay of Fundy” to see some of the largest tides in the world! 2) Check The Block Island Times for tides, and go to Smuggler’s to see if the actual high tide time matches what is predicted. To test this, go at a time near high tide and bring (or collect) a bunch of white stones you will recognize. Follow the water level as it reaches high tide by placing a stone on the beach where the water reaches every five minutes, and write down the time. When the tide starts to drop again, you will see the water level never reaches the highest stone again, and you can check your time against the prediction.

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