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Estuary In A Cup

In this activity, students explore density through investigating the properties of water in an estuary.

Saltwater is more dense (heavier), than freshwater because of the salt it contains. When river water pours out in an estuary and mixes with ocean water, the less dense (lighter) sits on the surface of the salt water until the ocean or tidal currents mix the two. 


  • Describe how salt affects the density of water.

  • List a variety of organisms which live in the marine and estuarine environment and describe how they are connected.

  • List and describe ways in which we all can be ocean stewards.


  • Per student:
    two clear plastic cups with about 1/2 cup of water in each
    one larger clear cup (empty)
    two colours of food colouring
    3 tablespoons of salt

Key Questions

  • Which liquid is on top? Why?
  • How can you help the two liquid mix faster?

What To Do

  1. Have students dissolve about 3 tablespoons of salt in one of the cups of water.
  2. Have them use the food colouring to make the saltwater one colour, and the freshwater a different colour.
  3. Students should now pour the saltwater into the empty larger cup.
  4. Have them use the eyedropper to drip the freshwater down the side of the larger cup onto the saltwater.
  5. Look through the side of the cup. Can you see layers? Which is on top, the salt or fresh water?


  • How much salt is in ocean water compared to freshwater?
  • Go for a beach walk and look at intertidal life, which is exposed to rain (freshwater) and salt water. What animals can you spot living on the rocks? What animals can you spot living in tide pools?
  • How do animals and plants survive the shifts of salt and fresh water in their environment?

Other Resources

NOAA’s National Ocean Service Education |  Estuaries