US Army Corps of Engineers
San Francisco District

Contact

Bay Model Visitor Center

2100 Bridgeway

Sausalito, CA 94965

Phone: 415-332-3871

Fax: 415-289-3004

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Water Quality

Grades: Fourth and up
Key Concept:
San Francisco Bay is one of the largest estuary in the U.S. and this estuaries important to many types of plants and animals. Scientists study and carefully monitor salinity levels in estuaries because they are so important to the lives of many plants and animals. In San Francisco Bay, the salinity content fluctuates because of the amount of fresh water flowing out into the Delta. However, since man has started taking salinity measurements from the Bay the concentration of salt water has continually increased. Thus making more and more of the Bay a salt water environment, rather than an estuary.

Objective:
(Students will be able to)

  • Explain the difference between salt and fresh water.
  • Describe and/or explain why the salinity concentration varies across the Bay, (Central Bay, South Bay, San Pablo Bay, Suisun Bay, and the Delta).
  • Under the density difference between salt and fresh water.
  • Understand the importance of monitoring salt water intrusion, and it’s effects on plants, animals and humans that live in and around the Bay.
  • Explain how the Oceans and Bays contribute to out water cycle.

Location:

Bay Model Visitor Center – Delta Area

Materials:

  • Constructed Hydrometer
  • 3-100ml Containers with different salinity levels (0, 8, 14%)
  • 3-100ml Graduated cylinders
  • 3 Medium sized beakers
  • Salinity Chart
  • Jar of fresh water colored red
  • Jar of salt water colored green
  • Colored stickers
  • Optional: Hard-boiled egg

Introduction to Activity (3 minutes)

Why is it important to determine the salinity of an area?
What are the differences between salt and fresh water?

Activity (5 minutes)

  • Get out the hydrometer
  • Explain density differences.
  • Get volunteers to fill up the cylinders.
  • Get readings off of the hydrometer, and place stickers on the chart.
  • Determine which solution had the most salt? And why?
  • Get out food coloring and add each to a brackish water solution.
  • Explain differences

Discussion (5 minutes)

  • Discuss estuaries and their importance as a unique environment.
  • Discuss the changes in salinity as you move up the river or towards the Golden Gate Bridge.
  • Discuss the role that oceans and bays play in our water cycle and watershed.

Background

  • Estuaries are important habitats that are essential; to the survival of many types of plants and animals. San Francisco Bay is a large estuary, and is not only important to plants and animals but to humans and the whole entire ecosystem.
  • Here at the Bay Model Visior Center Scientist monitored the salt water concentration and salt water intrusion; so that they could monitor and detect subtle variations in salt water concentrations that might effect the ecosystem of the Bay environment.

Discussion (5 minutes)

  • Ask: Why are estuaries important?
    Answer: Estuaries are important because they provide a brackish water habitat that many species of plants and wildlife depend on to survive. They are especially important to wildlife species because in areas where salt and fresh water mix there is an abundant amount of food. Because salt water is heavier it stays to the bottom and when the fresh water rises too the top it brings all the animal and plant material to the surface, so that species such as waterfowl can eat it. This is why Honker Bay is named for the Canadian Geese or “Honkers” because they stop over in this area to feed.
  • Ask: What water evaporates? Why?
    Answer: Fresh water evaporates because it is the water on the surface because it is lighter than salt water. The oceans also supply fresh water to our water cycle because salt does not evaporate. Salt dissolves in water to form a solution, but when the water evaporates the salt turns into salt crystals and is left behind. So, only fresh water evaporates into the clouds.
  • Do: Show them Honker Bay and explain to them this is the area where fresh and salt water mix, and that is why this area is so important.
    Read: “Historically the salt and fresh water mixed way down in San Pablo Bay, but since we started monitoring salt water intrusion the salt water has come all the way up thru Carquinez Straights and is now in Honker Bay. So protect our San Francisco Bay environment.

Method Introduction (3 minutes)

  • Ask: What are the differences between fresh and salt water besides the fact that salt water has salt in it? (Take a number of answers).
    Do: Show them each of the three solutions.
  • Ask: What differences can they detect? (Do not let them taste) - Salt water is cloudy - - smell
    Read: “Sure you can smell and see the difference but it is not very clear between the brackish (8%) and the slat water (14%). So, today you are going to learn how to tell the difference between salt and fresh water because of their different densities, with the use of a hydrometer. A hydrometer is a scientific tool for measuring the density of water, by floating higher in solutions than are more dense.”
  • Ask: Would it be easier for you to swim in a pool or the ocean if the ocean was as calm as the pool? Why? (Take a few answers).
    Read: It would be easier to swim in the ocean because of the increased density. Salt water has more little particles to keep you at the surface. The increased number of particles are ions created when the salt dissolved in the water.

Activity (5 minutes)

  • Do: Show the hydrometer.
    Ask: For 3 volunteers?
  • Do: Direct the students to each fill up the graduated cylinders, nearly to the top, with the three different solutions.
    Ask: How can you determine the difference between these three solutions with this highly scientific hydrometer? (Take a few answers).
    Ask: For 2 more volunteers.
    Read: Direct one student to take the reading off of the hydrometer right at the water level. Then ask the other student to put the sticker on the chart in the appropriate location.
  • Do: Repeat the above two steps for the other two solutions.
    Ask Which solution has the most salt in it? Why?
    Read: The reason that the hydrometer gave a higher reading in the salt water is because there are more particles or parts in the salt water, in the same volume of water, so the more particles do not let the hydrometer sink as far. Salt dissolves in water to create a solution, and that is why you cannot see the salt in the water.
  • Do: Show them the unknown.
    Ask: Who can determine the salinity of this solution?
  • Do: Choose another 2 volunteers. Repeat steps for other solutions to determine salinity.
    Ask: Which is heavier: Salt or fresh water?
  • Do: Add three solutions
    Red-Fresh
    Clear-Brackish
    Green-Salt
    Ask: Where should the green solution go if added to the clear solution?
  • Do: Add the green solution and show it to your students.
  • Do: Add the red solution.
    Ask What type of water is the red solution? Why?
    Ask: If fresh water is on top then what water evaporates and goes up into the clouds, only later to come back down?