From Mass. Bays Watershed Stewardship Guide
Adapted from A Raindrop Journey, by Barbara Waters


Salt Water Wedge


Materials Needed:

  • Plastic Box

  • Cool Water

  • Cool Salty Water

  • White Paper

  • Paper Cup

  • Small Stones

Grade Level: Grades 4-12

Time Required: 1/2 -1 hour

Disciplines: Science, Oceanography


  • To be able to explain why freshwater will stay at the surface while salt water will travel up a river along the bottom in a wedge because of density differences.

  • To be able to describe the water characteristics of an estuary, from salty ocean water, to brackish, to fresh.

  • To be able to identify estuary areas in Massachusetts where freshwater meets the sea.

Special Note:
To mix a seawater solution, add 35 grams of kosher salt (regular salt has additives) to 1 liter of water or approximately 1.2 ounces (2 scant tablespoons) of salt to 1 quart of water. To make a brackish mixture, halve the amount of salt Tint the salt water with the food coloring.


1. (Optional but recommended) Read the "Over the Wedge" poster and "A Raindrop Journey." These publications are available from the Mass. Bays Watershed Alliance (see below).

2. Place one end of the box on a small block Place a piece of white paper under the box and fold the extra paper up along the back to produce a white background.

3. Make several tiny holes in the bottom of the cup. Weight the cup with small stones and place at the lower end of the box.

4. Pour cool, fresh water into the box until it reaches close to the top of the cup. Allow the water to settle.

5. Gently pour the cool, salty, tinted water into the cup (do not overfill). 

Q. Describe what happens.

Q. Why? 



  1. Estuaries are an extremely valuable and productive area. Research and report on why they are so valuable and what sorts of resources are found there.

  2. Estuaries may be stratified in several layers due to different salinity gradients. Try the experiment, first using a brackish solution (with a red food dye) and then a salt-water solution (with a blue food dye) (make sure the water temperatures are all the same). What do you observe?

  3. Make a visual representation of salinity differences using two 16 ounce clear soda bottles. Fill one bottle completely to the top with freshwater and the other with salt water. Place some food dye in one bottle. With a piece of cardboard over the mouth. insert one bottle over the other. What happens? Try the experiment by changing the solution that goes on top. What happens now? Which is the halocline and which is the mixing process? The same experiment can be done with solutions of varying temperature to show thermoclines and temperature currents.

  4. Take a field trip to an estuary. Test water samples from various parts of the estuary with a commercial hydrometer which measures salinity (the saltier the water the more dense it is and the higher the wading on the apparatus). Or build your own homemade hydrometer. [Use a carrot or birthday candle with 1". of copper wire twisted around the bottom as a weight so that it just begins to sink if placed in freshwater. Using two-liter bottles with tops cut off for easy access or any two equal vessels, fill to equal levels with samples of salt and freshwater. Test your hydrometer -- either carrot or candle - in each. Since salt water is denser the hydrometer will float higher. Mark the level of water. Test your hydrometer in brackish water and mark the appropriate level.] Test the hydrometer with various samples from an estuary.

  5. Follow directions for "The Great Hydrometer Construction Contest!" in the April 1991 "Science Teacher"

Hydrometer Activity

Collect water to be tested for salinity and place in a narrow container. Float the hydrometer in the container and read the numbers where the narrow neck of the hydrometer breaks the surface of the water [meniscus]. The density [specific gravity] of the water determines the buoyancy of the hydrometer. The more salt in the water the more dense it is.


Fresh water has a salinity of 1.000.


Saltwater has a salinity of 1.20 to 1.30 depending on temperature and dilution of saltwater with fresh. In an estuary the saltwater from the open ocean is mixed with fresh water coming from the land.

Stewardship Guide developed by the Mass. Bays Education Alliance for teachers, under sponsorship of Mass. Bays Program and UMass. Extesion. Contact Faith Burbank to acquire a copy of the Guide and A Raindrop Journey, at or (781) 740-4913.