Pauline Misias- Density: Estuaries and Tidal Wedges

Title: Estuaries & tidal wedges (adapted from California Coastal Commission- “Waves, Wetlands, and Watersheds”)

Lesson concepts: In tidal estuaries, fresh water behaves differently from salt water due to differences in density of the waters. This difference in density is the driving force that creates tidal wedges.

California Content Standard:

8. All objects experience a buoyant force when immersed in a fluid. As a basis for understanding this concept:

8.a. Students know density is a mass per unit volume

8.d. Students know how to predict whether an object will float or sink.

Learning Objective:

Students will demonstrate why fresh water will stay at the surface while salt water will travel up a river along the bottom in a wedge because of density differences. Students will describe the characteristics of water in an estuary, from salty ocean water, to brackish, to fresh water.

Language Objective:

Students will use vocabulary words to explain what they observed and why they think this occurred.

Lesson:

Resources and Materials:

*Per group of students:

• Large, clear water proof box
• Tap water
• 1 quart room temperature salt water (made with sea salt and food coloring added)
• White paper
• Paper cup
• Small stones or pebbles

Required Background knowledge:

• Students will know an estuary is a semi-enclosed part of the coastal ocean where saltwater and fresh water from land mix. Estuaries behave in a way that allows water to flow in different directions.

Activities:

• Students will be organized into groups of 3-4 students
• Distribute worksheets to students and review the lab materials they will be using for this activity.
• Explain the experimental procedures students will need to be familiar with in order to be successful in this activity.
• Students will then begin the activity by:
  1. Putting a book under one end of the clear box, so that one side is about an inch higher than the other side.
  2. Students will make small holes in the bottom of the cup.
  3. Place small rocks or pebbles on the bottom of the cup, in order to weigh down the cup.
  4. Pour tap water into the box so that it is about half an inch below the edge of the box. Wait 3-4 minutes to allow the water to settle. (During this time students will write their hypothesis of what they believe will happen when they add the colored salt water to the cup).
  5. Each member in the group will slowly poor the room temperature salt water into the cup (be careful not to spill or to overflow the cup).
  6. After the salt water is added, get down low and look at the box from the side.
  7. Draw a diagram of your observations, and then write a description of what you saw. Be sure to include why you think this may be happening.
  8. Finally, students will think about what the model represents and where in the world water may behave in such a manner.

· When all groups have completed the activity, the class will come back together, and students will share their hypotheses, observations, and conclusions with the class.

· Finally, we will talk about tidal wedges and density differences in salt water and fresh water.

Application to the real world:

Students may not be familiar with importance of estuaries in their world. Many early societies were developed around rivers and estuaries because water was transported there relatively easily. In fact, seven of the ten largest cities in the United States are on large estuaries. San Francisco is an example of an estuary which students may have some prior knowledge about. Biologically, estuaries have great significance because they provide food for phytoplankton populations, which provides food for zooplankton, fish, and birds. Basically, estuaries are the breeding ground for many species.