Tide Status: It's Complicated

A few years ago, a colleague described her dismay when she learned that many of her undergraduate students did not know about tides. Most of them grew up within a few miles of the ocean, yet many did not realize that the seas rise and fall by a few feet twice each day. It was not that they were uncertain of the details; they really had no idea that tides are a thing.

One of the benefits of taking up rowing whaleboats has been that my own understanding and awareness of the coastal environment has greatly increased. I have started paying much closer attention to wind speed and direction, for example, and have learned some of the patterns of movement of other boats.

Yet the more I have learned about tides, the more I find I do not know. The harbor where we do most of our rowing is at the mouth of the Acushnet River -- where a small stream merges with the Atlantic Ocean. The interaction of currents and tides is complicated, and very important for understanding the behavior of our boats!

As I started to look into the relationship between tides and currents, I found that NOAA's maintains a series of FAQ pages on just that subject. I was particularly interested in the relationship between high and low tides and slack tides.

NOAA also offers a basic primer on tides -- Our Restless Tides methodically explains all of the factors that influence the timing and elevation of tides, from the sun and the moon to the orbits and declinations of each, as well as the shape of the ocean floor and the viscosity of water itself. It is fascinating to me that anybody understands all of these factors in as much detail as the folks at NOAA clearly do. They even know how much time is needed to develop a reliable record of tides in order to make local predictions: 18.6 years. Not 15, not 20, but 18.6!

All of which brings me to this map. The two markers -- less than 8 miles apart -- are located near the points where tides are calculated for the east and west ends of the Cape Cod Canal -- the world's largest sea-level canal (no locks). The locations are not precise -- the coordinates given on US Harbors are actually nearby points on land, but the locations are fine for making the point at this scale.


And the point is that since the entire canal is at sea level, water does not flow from one end to the other. In fact, prior to the canal's construction, small streams flowed from a point somewhere in the middle downward toward what are now the endpoints.

Several years ago, I was at the U.S. Army Corps of Engineers visitor center on the canal, where a former student happened to be a ranger. (Actually, the COE operates two visitors centers -- the one on the southeast side has more maritime information and navigational toys.) I asked him why the tide charts showed very different times at the two ends of the canal. The main reason has to do with sea floor topography, and the differences are so great in this area because the continental shelf is so much wider on the east side than on the west, and tides are affected as they cross this relatively shallow water.

The differences can be seen on today's (August 7, 2016) tidal chart as rendered by U.S. Harbors, a handy site for coastal information.

2021 Addendum 

I have long been curious about the two rivers (mentioned above) that were joined to create the Cape Cod Canal. As mentioned in the history section of the Cape Cod Canal entry on Wikipedia, these were the Manomet and Scusset Rivers -- indigenous names that are still used in other features in the region. I finally got curious enough to seek out a map! 

1891 map: Old Maps.com

I chose to show this 1891 map because it shows a very early segment of the Canal at the eastern end, but both rivers largely still intact. It is part of a series of maps depicting the area from 1753 (when the Manomet was called Herring Run on English maps) through 1917, when the Canal was complete.

Lagniappe -- Coffee Connection

It turns out that all of this is relevant to my favorite subject: coffee. My friend Byron was already known for making the best coffee in the world (literally: he won a world-wide Cup of Excellence) when he decided to involve the moon in his coffee production. Like most coffee experts, he was already carefully managing the sunlight on his shade-grown coffee, but he wondered if the moon's influence on water inside the coffee plant could in turn influence flavor. By concentrating sugars in the fruit, it turns out, it could do exactly this. Now his family is among the first to market specialty coffee to domestic retail customers in Nicaragua, and they call it Sol y Luna: Sun and Moon!