Lesson Plan: Salinity and Tides

This lesson uses web-based resources that are found within the theme with which this lesson is associated. The purpose of the lesson is to help you integrate these web-based resources into your curriculum.

Grade Level: 9–12

Subject Area: Earth Science


Standards Addressed:
National Science Education Standards

  1. Content Standard A: Science as Inquiry
    • Use technology and mathematics to improve investigations and communications.
    • Formulate and revise scientific explanations using logic and evidence.
    • Communicate and defend a scientific argument.
  2. Content Standard B: Physical Science
    • Motions and forces
    • Conservation of energy and the increase in disorder
    • Interactions of energy and matter
  3. Content Standard D - Earth and Space Science
    • Energy in the earth system
    • Geochemical cycles

Time Required:
3 (55 minute) class sessions plus homework

Lesson Goal:
The students will be able to analyze data to describe daily patterns of salinity changes in the estuary.

Learning Objectives:
Students will be able to:

  1. Analyze different forms of data and synthesize information to develop a hypothesis.
  2. Explain how tides and the geology of the estuary affect water circulation in an estuary.
  3. Describe daily patterns of salinity changes in the estuary.

Prerequisite Knowledge:

York River is one of several rivers flowing into Chesapeake Bay. As the nation’s largest estuary, Chesapeake Bay contains a diverse collection of habitats including oyster reefs, seagrass beds, tidal wetlands, sandy shoals and mudflats. Chesapeake Bay and York River illustrate the complexities of tidalvariation that respond not just to the gravitational pull of the sun and the moon, but also to the underlying topography of the bay and the dynamics of the estuarine river systems.

Chesapeake Bay Virginia National Estuarine Research Reserve has four sites on the York River, enabling research and education about the estuary, including extensive data from water quality stations and other observations by reserve scientists. In this learning activity, students use this multi-site system to explore tides and salinity from tidal freshwater to high salinity conditions along the York River estuary. Reserve components include Sweet Hall Marsh, Taskinas Creek, Catlett Island and Goodwin Islands.Rivers. Both rivers discharge into Chesapeake Bay.

Procedures/Instructional Strategy:
Make copies of Student Reading — Estuarine Tides, Student Worksheet — Salinity and Tides, and if you will not be providing computer-access to the data, Student Data Sheet — Salinity and Tide Data for York River. (Note that the data on the Student Data Sheet are for a specific date: March 21, 2007.)

Arrange for students to work with the animation and data, either in a computer lab or with a computer and projector.

Students Need to work in a computer lab or with a computer and projector Copy of the Student Reading — Estuarine Tides Copy of the Student Worksheet — Salinity and Tides Copy of the Student Data Sheet — Salinity and Tide Data for York River (if there is no computer-access to the data) U.S. Map and/or Google Earth Copy of Student Reading — Using Google Earth to Explore Estuaries (assuming you have computer access) - Find the tutorial in estuaries.gov, click under Teachers, Classroom Activities and find the tutorial.

Part 1 - Tides in Chesapeake Bay

  1. Introduce students to the Chesapeake Bay. If need be, use a U.S. Map to show students the location of Chesapeake Bay. Students can also learn more about the bay using Google Earth (refer to the Student Reading — Using Google Earth to Explore Estuaries for a brief how-to guide) or they can read more on the Chesapeake Bay Virginia NERR website: http://www.vims.edu/cbnerr/
  2. Using a computer projector for the whole class or letting students work individually or in teams in the computer lab, demonstrate the Tides in Chesapeake Bay website: http://tidesandcurrents.noaa.gov/ofs/cbofs/wl_nowcast.shtml
  3. Have students complete Part 1 of the Student Worksheet — Salinity and Tides.

Part 2 - Salinity as York River Flows into the Bay Here, you focus on salinity, helping students think, in a general way, about the salinity gradient in the York River as the fresh water flows into the salty bay.

  1. Make sure the students understand the location of York River in Chesapeake Bay.
  2. Have students complete Part 2 of the Student Worksheet — Salinity and Tides, labeling the York River map with “fresh,” “nearly fresh,” “fairly salty,” “close to seawater,” or “seawater.” If students have hands-on experiences in mixing fresh and seawater and/or have measured samples of fresh, brackish, and seawater, they can label the map with their best guesses about salinity, which will range from 0 (fresh) to about 35 parts per thousand (ocean).
  3. Have students compare maps in small groups and explain why they marked them as they did.
  4. Discuss daily and seasonal factors and Earth processes that affect salinity in an estuary.
  5. Have students read Student Reading — Estuarine Tides. (This can be assigned as homework.)

Part 3 - Interaction of Tides and River Flow With this part, students deepen their understanding of estuarine systems, focusing on the interaction of tides and rivers and how this affects salinity in the estuary.
Using a computer projector for the whole class or letting students work individually or in teams in the computer lab, demonstrate the animation of tides and salinity in York River at this website: http://web.vims.edu/physical/WEB/PRESNT/QTIME/Kimplot3.mov?svr=www.

  1. Make sure students are aware that the animation shows the change in salinity over a tidal cycle of 24 hours. Explain that the animation is not a representation of salinity changes for a specific date, but rather a model of what salinity distribution might be like in the river on any given day. Provide a general orientation about the animation for students:
    • The bottom and larger part of the animation shows horizontal distribution—salinity changing from upstream to downstream.
    • There are four reference points on the animation. Three are sites for which students will analyze salinity graphs in Part 2. (GLPT is Gloucester Point, #1 is Yorktown, and #3 is Clay Bank.)
    • The three images to the right show transverse slices of each of the three points—cut-away views of those locations—and show how saltier and fresher water is mixing from the surface of the water to the bottom.
    • The scale on the left shows the amount of salinity in parts per thousand (ppt). Students should generally know that moving from blue to red on the scale represents fresh to increasingly saltier water.
    • Students should also be aware that arrows on the image indicate the direction of water flow.
    • The hour on the animation indicates the time of day on the 24-hour clock.
    • The isohalines (lines on a chart connecting all points of equal salinity) help students determine levels of salinity.
  2. Encourage students to play this animation several times, looking for general patterns first, then at specific phenomena and distribution at specific places.
  3. Have students answer the first set of questions in Part 3 of the Student Worksheet — Salinity and Tides.
  4. Have students look at the cross-section views in the upper right of the animation, showing salinity with depth in the river, at the lines marked 1, 2 & 3, and answer the remaining questions.

Part 4 - Salinity as Measured by Water Quality Stations in York River Having seen what a theoretical salinity distribution can look like in the river, students now observe actual salinity data for a specific day at five different sites along the river. You can do this activity either using computer access to near-current data or using the prepared data graphs in the Student Data Sheet — Salinity and Tide Data for York River.

  1. If you use the computer access to data, follow the instructions in the Student Worksheet — Salinity and Tides. If you use the prepared graphs, hand them out to students. Students will:
    • Make predictions about how fresher and saltier water will mix, and how salinity changes throughout the day will differ from site to site.
    • Select data for the same date at each station. Students may select a date of their own choosing, but for the purposes of this initial activity, it will be best for the whole class to choose the same date for the sake of consistency when they are discussing results.
    • Observe graphs of salinity data for that day at each site. It will be helpful for students to print out graphs for each site so they can compare changes from site to site, over time.
  2. Encourage students to correlate the salinity graphs at the five sites with the generalized distribution shown in the animation. Students should pay particular attention to the graphs of Gloucester Point, Yorktown, and Clay Bank because this is the area marked by the three reference points in the animation.
  3. Have students answer the questions in Part 4 of the Student Worksheet — Salinity and Tides.


  1. Discuss the following:
    • How do the changes at each monitoring station compare with changes at those same areas in the animation?
    • Name several factors that determine why salinity changes are different depending on your location within the estuary.
  2. Ask small groups to use their handouts to answer this question. Collect this assignment and use it as a final assessment. Imagine that an intense rainstorm dumps 3 inches of rain over the entire Chesapeake Bay region. Predict how the salinity would change at all four stations in the bay for a period of 24 hours after the storm ends. Supply a graph and an explanation of what you might expect to see at each station.

Have students access other data from the VIMS site to see how factors such as precipitation and temperature might have affected salinity on that date.

Classroom Resources:
Computers with Internet access for students

Lesson Plan File:
(entire word document containing complete lesson plan and supporting attachments)
Download Here (pdf, 554)

Student Work Description:
This is an image showing a poster of salinity and tides in the Chesapeake Bay and York River.

Sample of Student Work:
Download Here

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