Ocean Tides and Currents 2


Here’s a question for you: Why should a swimmer give a hoot about ocean tides?  Well, if you ever want to swim at any of the hundreds of great spots along the Washington, Oregon or California coasts, it might be worth a few minutes of your time to know about local tides.  Beyond that, be aware that oceanic tides also affect the constantly changing levels and currents in every river and bay, often a hundred miles or more from the coast.

Access to that kind of information was of immense value to me (and to my swimmers) during the three decades that I directed swims and triathlons in San Francisco Bay.  A wrong tidal call would have put many swimmers in danger.

Here is how to get that information.  First, there is an important distinction between tides and currents.  Oceanic tides refer to the up and down motion of the sea.  Tides constantly vary from time to time and from place to place.  The moon and the sun exert a gravitational pull on our planet, causing the water to follow, with the pull of the moon being strongest because it is so much closer.  The greatest tidal movement occurs when the sun and moon are in alignment, that is, at the new moon and the full moon.

When we speak of currents, we are usually talking about the horizontal movement of the water, and in the ocean, currents normally result from tidal movements.  In rivers the water running downhill from the mountains to the sea give us our currents.  Near the sea, river currents are affected by the rise and fall of the tides.

This can be a very complex study.  Not to fear!  Our federal government has been studying tides and currents for many years.  They are constantly gathering data to be able to tabulate tidal predictions for years into the future.  The arm of the government charged with that service is NOAA.  Access to the data is free and relatively easy.  I will take you through two examples.  Both look at predictions for Labor Day, 2020.  The first example is at the mouth of the Columbia River.  The second is in the Columbia River at Vancouver, WA.  Follow the steps below.

  1. Go to https://tidesandcurrents.noaa.gov/tide_predictions.html?gid=1409. That will get you to the NOAA tide and current prediction site.
  2. Under COLUMBIA RIVER select COLUMBIA RIVER ENTRANCE from the long list of locations. That will give you ocean tides for that location near Astoria.  (The tide times at that location will not be very different from locations up and down the coast.)  What will appear are the tides for that day, at that location.  It will appear in the form of a sine curve plotting the height of the tide against the time of day.
  3. We are looking for a specific date, September 7. Slightly below the sine wave, on the left is a pair of boxes where you can enter any date you want, or any series of days.  Put in September 7, 2020, in both boxes.
  4. Now go to the right and click on the blue box, PLOT DAILY. A sine wave will appear in a few seconds that tells you the values of the tides on September 7, 2020, at the entrance of the Columbia River.  (High tide of 5.51’ at 4:30 am; Low tide of 1.53’ at 9:45 am; etc.)

Our second example is for the tides in the Columbia River at Vancouver on Labor Day, September 7, 2020.

  1. Go back to the OREGON listings.  Under Columbia River, go down to Vancouver and click it.
  2. The tides for today’s date will appear in sine wave form. Go below to the date boxes on the left and enter September 7 in both boxes.
  3. Go across the page to your right and click PLOT DAILY. A sine wave will appear for September 7, with the tides at Vancouver, WA.  (Low tide of 0.67’ at 5:15 am; High tide of 1.88‘ at 9:45 am; a second low of 0.26’ at 4:30 pm; a second high of 2.88’ at 10:00 pm.

Now you know the values of the high and low tides.  But that is only part of what you need.  What you really want to know about are the currents.  That is a more complicated bit of information, but we have a few clues about how to determine the currents.

Keep in mind that the River always wants to flow out to the sea.  When the ocean tide is rising, it wants to push the river back.  The further you go from the sea, the less will be the effect of the ocean tides.  But those tides affect the river up to and beyond Vancouver.  Now take another look at the sine curve for that day at Vancouver.  Between the early low at 5:15 am and the early high at 9:45 am, there is a difference between the high and low tides of 1.21 feet.  Note that the curve is not very steep at that point.  The indication is that there will be less current as we approach the flat part of the curve at 9:45 am.  Then think about the time of year.  In the spring there will be a greater effect of snow melting in the mountains.  By September that effect will lessen.  And the water will be warmer.

In some places NOAA gives much more specific information regarding currents.  San Francisco Bay is one such place.

I hope that I have not confused things.  All it takes is a bit of practice.  Give it a try.  Remember to never swim alone.  Also keep in mind that the sea temperatures at the Oregon coast are usually quite cold, often in the fifties.  Hypothermia is a real threat.

 

Editor’s note:  For over 30 years Joe organized and ran the annual “Escape From Alcatraz Triathlon,” and selected dates for other events in San Francisco Bay.  He selected these dates according to when the tides would be best.  He also wrote about tides in a national kayaking magazine.


Leave a comment

Your email address will not be published.

2 thoughts on “Ocean Tides and Currents