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Lesson 4

The Layered Look:
Where Fresh and Salt Water Meet
Key Concepts
  1. Fresh water tends to flow above salt water.

  2. Stratification tends to occur in estuaries where fresh water meets salt water.
  3. Puget Sound can be described as an estuary.

  4. Point and non-point sourcrs of pollution may enter the Sound through fresh water run-off.

  • Students will predict what happens when fresh and salt water meet.

  • Students will create an experimental model of fresh water flowing into salt water.

  • Students will apply the results from their model to make inferences about data presented photographically.

Process Skills
communicatinginferringinterpretating data

Kit Materials
  • 32 copies of Activity Sheet #4A - The Layered Look
  • 2 copies of a photograph showing muddy, fresh water on the surface of Puget Sound near the shore
  • Transparency #4A - Circulation of Puget Sound
  • The Surface Water Video
For each group of four students:
  • one clear plastic container
  • food coloring
  • one paper cup
  • salt
  • straight pin
  • teaspoon
Non Kit Materials
  • tap water
  • paper towels
  • VCR and monitor

rainfall that is not absorbed by the soil and eventually reaches surface water bodies. Surface water pollution can occur when this rain runs off the land, carrying pollutants on the ground into local rivers and lakes.
a semi-enclosed arm of the sea where incoming sea water is diluted with fresh water coming from rivers draining the land
surface water
all the water we see in oceans, lakes, rivers, streams and wetlands
refers to the concentration of salts dissolved in water
point source pollution
a source of pollution froma single source of conveyance, such as the discharge pipe from a sewage treatment plant or a factory
non-point pollution
pollution that enters water from dispersed and uncontroled sources (such as carrying pesticides or fertilizer residues, or failing septic tanks) rather than through a pipe

Teacher Background
Freshwater flows into Puget Sound sub-basins Water movement plays an important role in shaping both the beaches and the health of Puget Sound. Tides circulate water in the Sound, while rivers add fresh water and the material it carries. The fresh water flows reaching each of the four major sub-basins of Puget Sound differ because of the size and nature of the watershed. Sixty percent of the total freah water entering the Sound flows into the Whidbey Sub-basin from the drainages of the largest rivers in the area, the Skagit, Stillaguamish, and Snohomish. These rivers collectivly drain about 50 percent of the Sound. The main Sub-basin receives 20 percent of the fresh water entering the Sound from the Puyallup, Green/Duamish, Sammamish, and Cedar Rivers. No major rivers flowinto the Hood Canal Sub-basin, but Hood Canal receives 10 percent of the fresh water entering the Sound through minor rivers (Snohomish, Dosewallips, Duckabush, Hamma Hamma, and an unnamed offshoot of the Skykomish River). The Southern Sub-basin receives less than 10 percent of the drainage into the Sound even though it has a large drainage area (Burns, 1985). It is fed mostly by small rivers and streams (the only major rivers are the Nisqually and the Deschutes).
Estuarine Processes
Flow Patterns Estuaries have a distinctive pattern of wter movement created by the action of the tides and the presence of fresh water. Since estuaries like Puget Sound are extensions of the ocean, they are affected by the tides which pump large volumes of water back and forth, in and out of the Puget Sound basin. Fresh water entering the Sound from streams or rivers is lighter or less dense than saltwater and tends to float and flow on top of the seawater. As it does this, some of the saltwater is mixed up with the fresh water, creating a brackish (less salty) layer at the surface (about 30 to 90 feet or 10 to 60 meters deep in various parts of the Puget Sound region). This layer flows seaward under the force of gravity, eventually reaching the Pacific Ocean. To replace the waewater in the deep layer which was mixed into the surface layer, more seawater is drawn into the estuary from the ocean. This characteristic net estuarine circulation (seaward at the surface and ladward below0 exists throughout Puget Sound and the Straits of Georgia and Juan de Fuca.

The two-layered estuarine pattern described above is complicated in Puget Sound by the presence of various islands and channels. For example, the blocking effect of Vashon Island disrupts the two-layered flow pattern in East Passage, where currents at all depths are generally to the south, and in Colvos Passage, where currents are generally to the north (see Transparency #2A). The relatively shalow sills that divide the sub-basins of the Sound also disrupt the two-layered flow pattern. Implications of these complicated flows are that as much as one-half to two-thirds of the outflowing surface water from the Puget Sound Main Sub-basin may be diverted before going through Admiralty Inlet (Ebbesmeyer and Barnes, 1980). Researchers have estimated that as much as one-third to one-half of the outflowing water makes a submarine return trip through the depths of the Main Sub-basin instead of exiting to the Strait. More recent modeling efforts appear to indicate that about one-quarter of the outflowing water return to the Main Sub-basin (Cokelet, 1987). The same type of complicated flow pattern can be inferred in the other sub-basins set off by sills (Ebbesmeyer and Barnes, 1980).

Exchange with the ocean What are the implications of these flow patterns? An important physical characteristic of an estuary is its ability to exchange water with the open ocean. Exchange helps cleanse the deep basins of the Sound and prevent them from becomming naturally stagnant from organic decay. Exchange also plays a critical role in governing the fate and effects of contaminants that enter Puget Sound. It has the potential to carry dissolved waste products out to sea, However in Puget Sound south of Admiralty Inlet, the diversion and remixing of surface water mentioned above limits the amount of exchange with the open ocean.

Puget Sound is not a pipe or an open drain that will carry dissolved or suspended contaminants directly out to sea. The process is short-circuited by the recirculation of surface water and the settling of sediments. For example, fresh water on the surface of the Main Sub-basin takes about a week to get from the mouth of the Duamish River to the Admiralty sill. Then, much of it spends about 10 days going back to its starting point. It must make the trip twice, on the average, before reaching the Strait of Juan de Fuca. Computer simulations of water movements have shown that after three months, only half of the water released in East Passage escaped to the open ocean. Implications of this estuarine recirculation and resulting slow exchange with the ocean that any contaminants carried in the surface water remain in the area for some time, and can be spread throughout the aSound before exiting to the Strait or settling out. In addition, any contaminants or items that wash up onto or adhere to shorelines (e.g., plastic garbage and oil) are likely to never leave the Sound.

For additional information on Puget Sound circulation, see State of the Sound 1988 Report, Puget Sound Water Quality Authority, from which the above material is taken.

Procedure - Lesson 4 Helpful Hints
This lesson contains two activities. Both may take 30-45 minutes to complete.

Activity 1 - Where Fresh and Salt Water Meet

  1. Review the names and locations of the major rivers that enter Puget Sound. Reinforce that the rivers bring fresh water to the salty water of Puget Sound.

    "What do you think happens when fresh water from rivers and streams flows into the salt water of Puget Sound?"

    Record students' responses for later reference. Explain that this activity will explore what happens when these waters meet.

  2. Distribute to students Activity Sheet #4A - The Layered Look. Point out the diagram of the equipment setup. For best observation, remind students to gently pull the pin out of the cup.

    "What do you think you will see when the fresh water runs into the salt water?"

    Have students record their prediction on the data sheet.

  3. Divide students into working groups of four and distribute the materials required. Display the two copies of the photograph showing muddy, fresh water on the surface of Puget Sound near the shore. Students will observe this photograph to complete the last "Analysis and Interpretation" question.

  4. When all groups have completed the experiment, examine the list created in Procedure #1. Compare observations of the model and students' ideas of what might happen when salt and fresh water meet.

    "What do you think happens to the fresh water that flows onto the salt water?"

    Discuss students' ideas.

Activity 1 - Moving Water
  1. Dislpay transparency #4A - Circulation in Puget Sound.See teacher background for a detailed interpretation of this diagram.

  2. Explain that Puget Sound could be described as an ESTUARY.

    "The fresh water entering the Sound brings more than just fresh water to dilute the salt water. What else does the fresh water carry to the Sound?"

    The fresh water may carry:
    1. materials that end up as sediment on the beaches or in the bottom of the Sound (students remember about Lesson 2);
    2. nutrients and organic matter from the decay of plants and animal matter in the watershes;
    3. pollutants from the watershed that affect water quality and the health of sediments. Fresh water run-off may carry excess nutrients, bacteria, or toxic chemicals, including pesticides and herbicides.

  3. Show the videotape, Surface Water (9 minutes). Discuss the sources of point and non-point pollution.
Please look ahead to Lesson 5: "Creature Features" which introduces research reports students will complete. You may choose to present Lesson 5 before this lesson to allow more time for completion of the student reports.

  1. You may choose to have students work together in their small group to begin answering the "Analysis and Interpretation" questions, or work onthem together as a class when all groups have completed the activity.

The main idea to convey is:
Fresh water entering the Sound is lighter (less dense) than the salty ocean water and tends to float and flow over the top of salt water. As it does, some of the salt water is mixed up with the fresh water, creating a less salty (brackish) layer at the surface (30-190 ft. in various parts of the Sound). This brackish surface layer flows seaward, eventually reaching the Pacific Ocean. To replace the surface layer, more deep sea water is drawn into the estuary from the ocean. Estuarine circulation = seaward at surface and landward below.
Ocean        Sound


  1. An ESTUARY is a semi-enclosed arm of the sea where seawater, brought in by the tides, is diluted with fresh water coming from the land.