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The Hudson's Ups and Downs

Educator Lesson Plan *DRAFT*

Students will interpret line graphs of Hudson River water levels to learn about tides and tidal cycles in the estuary.

Objectives:

Students will read line graphs to:

  • see that tides cause regular changes in water level along Hudson River estuary;
  • observe that high tides and low tides occur in predictable cycles;
  • understand that high and low tides occur at different times in different places along the Hudson estuary; and
  • explore how weather can affect water levels and tides.

Grade level:

Elementary (Grade 5)

Subject Area:

Math, Science

Standards:

Mathematics, Science, & Technology Standards 3, 4

Skills:

  • Use line graphs to analyze patterns observed in the physical environment.
  • Use line graphs to compare and contrast data and events.

Vocabulary:

estuary, high tide, low tide, sea level, tidal cycle

Duration:

Preparation time: 5 minutes

Activity time: 40 minutes

Materials:

Each student should have:

Background:

The Hudson's surface is roughly at sea level from New York Harbor to the dam at Troy, and is influenced by ocean tides over that distance. This lesson explores the cycle of high and low tides but not their causes, which involve the gravitational attraction between the moon and earth and their relative positions-topics difficult for elementary students to comprehend. That said, a brief explanation will be useful to teachers covering tide cycles.

In the oceans, gravitational attraction causes water to bulge up towards the moon. This bulge remains positioned under the moon as the earth spins around. Thus, while strollers on the Jersey shore watch the moon rise, they are being carried into a mound of water, evident in the rising tide lapping at their feet. This bulge enters the Hudson and rolls upriver, causing a high tide here. Hours later, after the Atlantic coast has passed under the moon and reached the backside of the bulge, the tide starts to fall.

For reasons best left to outside reading, there is a second bulge on the side of the earth opposite the moon. In between the bulges, ocean levels are lower, causing low tides. So in the 24 hours it takes the earth to rotate once on its axis, the Atlantic coast will usually have two high tides and two low tides. Actually, because the moon revolves around the earth, a complete tidal cycle takes 24 hours and 50 minutes. Imagine yourself checking your watch as the earth spins you directly under the moon. Then, as you wait for the earth to spin full circle, the moon doesn't stand still. It moves ahead, towards the east as you view things, so you must wait 24 hours plus 50 minutes to be directly under the moon again. Thus the timing of a given tide falls back 50 minutes each day, on average. For example, if low tide on Monday morning is at 9:00, low tide Tuesday morning would be at 9:50.

Activity:

  1. Discuss what tides are, perhaps by having students recount visits to the ocean.
  2. Relate what the line graphs show to the reality of water levels rising and falling.
  3. This activity is best done in class with the teacher available to provide assistance.

Assessment:

  • Have students share answers to questions, or collect and grade sheets.
  • Make up similar questions for quiz.

Answers:

1. In Albany, river is at sea level; rises & falls in regular pattern
2. Tide low at 10 AM 3/9
3. Tide high at 3 PM 3/8
4. High & low tides are not all equally high or low
5. 2 low tides & 2 high tides each day
6. About 6-6½ hours
7. About 12-13 hours
8. About 6-6½ hours
9. About 13 hours
10. First high 1-2 AM; first low about 7-8 AM
11. Low tide in Albany
12. Tide high in Poughkeepsie
13. About 7 hours
14. Rose about 4 feet
15. Heavy rains caused the Hudson to flood at Hadley and Albany. Both high and low tides were much higher than normal, but the timing of the tide cycle was mostly unchanged.

Resources:

http://ny.water.usgs.gov/projects/dialer_plots/saltfront.html
The U.S. Geological Survey's Hudson River Salt Front website provided data for the graphs in this lesson. The site has real-time data for Hastings on Hudson, West Point, Poughkeepsie, and Albany plus links to sites on the upper Hudson and on tributary streams. You can generate custom graphs using this site. Click on the 15 -minute data link for one of the four estuary sites, and then scroll down to this table. Select the parameter to display, the output format, and the period of time to show-1 to 31 days prior to the time of your request.

On March 26, 2007, selecting "Elevation above NGVD" from the available parameters (NGVD is an approximation of sea level), "Graph" from the output format list, and 7 days of coverage, and then clicking on "GO" produced this graph of high and low tides.

For predictions of high and low tides, visit http://tidesandcurrents.noaa.gov/tides07/ posted by the National Ocean Survey. (After 2007, change 07 to 08, 09, etc. in the URL.) Once the page has opened, select New York. From the list of waterways on the next page, click on Hudson River. In the list of locations then presented, choose the one desired and-in the far right hand column-click on predictions. Keep in mind that these are only predictions; weather conditions may affect the actual tide times.