- Teacher demonstration of salt and fresh water mixing and stratification
- Students use Hudson river data to determine the impact of tides on salinity
- Students discuss their graphs
Tides in the Hudson
Students will know how tides affect the Hudson River and be able to create a graph showing a two-day pattern of tides in the river.
- 2 clear plastic containers
- 2 siphons (or spoons if you want to do a smaller demonstration)
- clear fresh water
- fresh water with drops of food coloring
- clear salt water
- salt water with drops of food coloring
For student groups:
- Copies of worksheet
- Computers with Excel or extra paper on which students may draw graphs
You should make the salt water ahead of time; use Kosher salt to eliminate the cloudiness. Mix 35 grams of Kosher salt to 1 liter of water for salt water that is 35 ppm. If you don’t have a good balance, mix 1 gallon of water with 1 cup of deionized salt.
Show a picture of the Hudson River watershed, and ask students to identify the water bodies shown. Ask: What kind of water would you expect in the ocean? In the river? In between? Ask students what they think happens when fresh and salt water mix. Define estuary if necessary. Tell them that you have a large container of fresh water and that you are going to add a large amount of colored salt water. Have one student come up and taste the salt water to prove that it’s salty. Using the siphon, carefully add the colored salt water to the container of fresh water, keeping the siphon near the bottom. Ask the students to describe what they see happening and whether this matched their predictions. Repeat the test with the fresh water containing the food coloring. Ask students why you have to repeat the test this way (to eliminate the possibility of food coloring as a variable). Then ask the students why the salt water stayed at the bottom of the container.
Students will now use data showing the salinity levels over a week-long period at West Point on the Hudson River to think about the tides. There are several versions of this worksheet available for you to use, depending on the graphing abilities of your students and what skills you are focusing on. Ideally, the students should graph these data, using time on the x-axis and salinity along the y-axis. If students have access to Excel, they can use the data set titled “Salinity from West Point” and create the graph in Excel. If students don’t have access or don’t know how to use Excel, they can use the attached worksheet and graph the data by hand. When the students have finished with their graphs, they should compare them with those of other students in the class. The class should see a four day period of salinity changes, as shown in the worksheet with the graph embedded.
Ask: Why did the salinity change? If students are lacking information about the ocean, this might be a good time to explain that the salinity of the open ocean is around 35 ppm, or 35000 ppt. Ask: What is the level of salinity of fresh water? Students should be able to explain why the levels of salinity fluctuate throughout the course of a day, a week, and a month.
Tides are caused by the interaction between gravitational and centrifugal forces, with the strongest tides resulting from the pull of the moon. Water levels rise on the side of the earth nearest the moon and on the side opposite the moon, causing a ‘bulge’ of water around the earth. Since the moon circles the earth every 24 hours and 50 minutes, an observer standing at one point on Earth would see two high tides and two low tides during this period. These tides are called semidiurnal. The Hudson is open to the ocean, and semidiurnal tides travel up the Hudson all the way to the Troy dam. Tides make the water in the Hudson slosh back and forth, much as you would see in a bathtub. The river runs north for six hours and then runs south for six hours. However, freshwater from rivers can change the flow of the river and even prevent the tidal push during very heavy runoff. The salt front, which is the leading edge of the diluted sea water, is usually below the Tappan Zee Bridge, but it sometimes comes to Poughkeepsie or even further, depending on weather conditions. Salt in the water could be dangerous for the communities along the Hudson that obtain their drinking water from the river. It is therefore very important for riverfront communities to monitor the location of the salt front.
- Depending on students’ understanding of tides and tidal cycles, it might be a good idea to complete an additional activity on tides, such as graphing the USGS’s Salt Front Data which is available at http://on.doi.gov/125CCjI.
- Students can use the “Tides in the Hudson” reading to learn more details about the nature of tidal change in the Hudson River.
- Students can also graph data from the annual “A Day in the Life of the Hudson River” event (aka ‘Snapshot Day’), found here: http://bit.ly/13dX6Gf .
Evaluate: Assess student understanding from graph creation and answers to questions on Tides and Salinity Worksheet. Students should also read the Related Reading: “Tides in the Hudson River” and answer the student questions.