So What is this Salt Wedge, anyway?

In estuaries, salt water meets fresh water. Let's make a miniature estuary and study what happens!

Our study will be scientific - we'll make and record careful observations, try to explain what we see, use our explanations to predict what else might happen, then test our predictions. TEACHERS: practice this before having your students try - it takes patience to get really good results, but it's worth it!


We'll need:
  • salt (NaCl) - the cheapest kind often has the best crystals!
  • water
  • food coloring
  • 2 clear plastic glasses
  • PENCIL and PAPER!! (recording is important!!)
  • optional magnifying glass or microscope
  • paper towels in case of a spill


    • fill one glass about 2/3 way with water
    • put NaCl in the other glass, about 1/8 full or so
    • take a pinch of the salt, look at it closely (microscope helps!)
    • record your observations (sketch and/or verbal description)
    • add water to the salt, about 1/4 glass, stir/slosh til dissolved
      - it's OK if some salt remains. Describe what happens
    • If you have a microscope (40x is good), put a drop of water on some
      salt grains and watch patiently under the scope - describe!
    • add some food coloring to make the very salty water dark
    • very gently pour the salty water into the fresh water
      (it helps to tilt the fresh water glass to slow the pouring)
    • observe and record the results!

    We've got some explaining to do!

    At least 3 good questions should come from your observations:
    • Why are the NaCl crystals shaped like cubes?
    • How did the salt disappear when water was added?
    • Why does the salty water go to the bottom?
    When ocean water comes into an estuary with the tide, it tends to settle to the bottom just like our model. As it pushes its way in, it usually gets a wedge shape - hence the title of this Web Site!