The Hudson's Ups and Downs
Students will interpret line graphs of Hudson River water levels to learn about tides and tidal cycles in the estuary.
Students will read line graphs to:
- examine how tides change water levels 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;
- explore how weather can affect water levels and tides.
Elementary (Grade 5-7)
Mathematics, Science, & Technology Standards 3, 4
- Use line graphs to analyze patterns observed in the physical environment.
- Use line graphs to compare and contrast data and events.
Preparation time: 5 minutes
Activity time: 40 minutes
Each student should have:
- Worksheet: The Hudson's Ups and Downs (PDF) 330 KB
- Pen or pencil
It would be helpful for the teacher to have:
- A jump rope or other length of rope
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. These tides are important to the movement of ships, the plans of kayakers and anglers, the distribution of aquatic plant communities, and many other aspects of economic, recreational, and ecological activity along the river.
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 and greatly simplified explanation may be useful as background.
Imagine the earth as an idealized ball covered with water at the same depth all around. The moon's gravitational attraction shapes this idealized ball into an ovoid, an egg-shaped object. One of the oval's elongated ends is directly under the moon; the other is on the opposite side of the earth. These elongated ends can be thought of as bulges. While both earth's crust and the oceans bulge, the effect is much greater in the water. These bulges are high tides.
Now put this picture in motion. As the earth spins on its axis, the bulges remain in position under the moon, and are experienced as two daily high tides along the Atlantic coast. 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, we will usually have two high tides and two low tides. Actually, because the moon revolves around the earth, a complete tidal cycle takes more than 24 hours. Imagine checking your watch when you are directly under the moon and then waiting for the earth to spin full circle. In that time the moon doesn't stand still. It moves ahead towards the east, so 24 hours plus 50 minutes go by before you are 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.
While the above theoretically explains the forces that produce tides, the response of actual oceans, divided up into basins separated by continents, depends on the shape of the perimeter and sea floor of these basins. The bulges do not literally move across the oceans in two massive waves. An explanation of these tidal dynamics goes beyond the space available here.
- Discuss what tides are, perhaps by having students recount visits to the ocean.
- Relate what the line graphs show to the reality of water levels rising and falling.
- This activity is best done in class with the teacher available to provide assistance.
- Have students share answers to questions, or collect and grade sheets.
- On a classroom computer or Smartboard, visit a Hudson River remote sensing website and use current water level data (see below) to have students identify high and low tides.
Available in the PDF version of this teacher's section and in the package that bundles all of the science lessons.
- dam: a barrier built across a stream
- estuary: a body of water in which fresh and salt water meet
- high tide: highest water level in the tidal cycle
- low tide: lowest water level in the tidal cycle
- sea level: the average height of the ocean
- tidal cycle: the repetitive rise and fall of the ocean's surface over a 24-hour period
- tides: the alternating rise and fall of the surface of the ocean and bodies of water closely linked to it
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 Poughkeepsie and Albany plus links to water level gauges on the upper Hudson and on tributary streams. Such data are also available from HRECOS (the Hudson River Environmental Conditions Observing System), a network of real-time monitoring stations at sites on the estuary from Albany to New York City. Its sensors usually take measurements every 15 minutes, and offer a range of water and weather information. Visit these two sites using the Links Leaving DEC's Website section in the right-hand column of this page. You can generate custom graphs using these sites. Basic instructions for creating such graphs are available in the Teacher Section for "The Hudson's Ups and Downs" (PDF) 450 KB
For predictions of high and low tides, visit the National Oceanic and Atmospheric Administration's Tide Predictions page for New York (see Links Leaving DEC's Website). Scroll down to the Hudson River predictions, then click on the location desired to see the current day's predicted tides displayed in both a graph and a table. To see predictions for other days, select dates using the drop-down menus below the graph and then click on Submit. Keep in mind that these are only predictions; weather conditions may affect the actual tide times and heights.