Environmental Protection Agency Narragansett Bay Commission University of Rhode Island Office of Marine Programs

Estuarine Science

Tides

1. What are tides?
2. How do we measure the tides?
3. What are the tides in Narragansett Bay?
4. What can we learn by measuring tides?


1. What are tides?

Tides are the rising and falling of the water level in the ocean. Tides are caused by the gravitational attraction of the moon and the sun. The moon has the single greatest influence on the tides. Tidal forces cause the water level to rise and fall in a cyclic manner. To make the water level rise and fall, a lot of water gets moved around and this can cause tidal currents.

Tides in Narragansett Bay are semidiurnal, meaning that there are two high tides each day and two low tides each day.

The tidal range changes depending on the relative positions of the moon and the sun. If the moon and the sun are acting together, we get a higher than normal tide called a spring tide. When the moon and sun are acting opposite to each other, we get a lower than normal tide called a neap tide. There are other astronomical factors that can change the tidal height.
For an in-depth explanation of how the gravity of the moon and the sun cause tides, see this web site: http://www.co-ops.nos.noaa.gov/about2.html

In addition, there are local influences on the tides. Some of those factors are:

  • Shape of the land
  • Shape of the ocean floor (bathymetery)
  • Depth of water
  • Restrictions to flow (narrow inlets to bays, etc.)
  • Local winds


1. What are tides?
2. How do we measure the tides?
3. What are the tides in Narragansett Bay?
4. What can we learn by measuring tides?

       


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