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Tracking the Salt Front

Lesson Plan

Students will use Hudson River salinity data to practice math skills as they track movements of the salt front in response to storms and other weather events.


Students will solve word problems that require them to:

  • Understand interactions between salt water entering the river from the sea and fresh water entering the river from its watershed;
  • Interpret data displayed in graphs and on maps;
  • Add and subtract river mile data to track salt front movements.

Grade level:

Elementary (Grades 3-5)

Subject Area:

Math, Science

New York State Learning Standards:

Mathematics, Science, & Technology Standards 3, 4


  • Use graphs and maps to see patterns and relationships observed in the physical environment.
  • Use whole numbers to identify locations and measure distances.
  • Add and subtract whole numbers.
  • Apply mathematics in real world settings.
  • Reason mathematically.


Preparation time: 5 minutes
Activity time: 1 hour (about 30 minutes for each worksheet)


Each student should have:

  • Worksheet: Tracking the Salt Front (PDF) 110 KB
  • Hudson River Miles map (PDF) 190 KB * Note: This is set for legal (8.5 inches x 14 inches) paper to maximize legibility, but it will work on letter (8.5 inches x 11 inches) paper as well. To obtain legal-size copies, you may need to use the options in your computer's print menu to specify printout on legal size paper.
  • Pencil


The lower Hudson is an estuary, where fresh water from the river's watershed and salt water from the Atlantic Ocean meet and mix. Seawater entering the Hudson is diluted by fresh water; its leading edge-the salt front-is where the concentration of chlorides (mostly sodium chloride, like table salt) reaches 100 milligrams per liter (mg/L). Many people assume that tidal currents bring salt water into the river. However, they have only minor impacts, moving the salt front a few miles back and forth with each tide cycle. So how does seawater travel 50 miles or more up the Hudson?

Imagine an aquarium divided vertically by a panel separating salt water (colored green with food coloring) on one side from clear fresh water on the other. If the panel is removed slowly, the green salt water, being denser, will flow under the fresh water, which will in turn flow over the salt water remaining on the opposite side. Some mixing will occur, but two layers should be visible.

This phenomenon occurs in the Hudson, where it is called estuarine circulation. Tidal currents and wind tend to mix things up, blurring the layering, but denser seawater does push upriver under fresher water headed downriver at the surface.

How far upriver will the salt water go? That depends on the volume of freshwater runoff from the watershed. After heavy rains, runoff pushes the salt front seaward. During dry spells, seawater pushes further upriver. The salt front usually ranges between Newburgh and the Tappan Zee Bridge. Droughts may allow it to reach Poughkeepsie; major rainstorms may force the front downriver to Manhattan.

The front's location is given in Hudson River Miles (HRM), starting at the southern tip of Manhattan. This spot, called The Battery, is HRM 0. Ocean tides influence the Hudson north to the Federal Dam at Troy (HRM 153).


  1. In preparation for this lesson, have students do the Readings in Hudson River Natural History titled "From the Mountains to the Sea."
  2. Discuss the concepts of estuary and salt front.
  3. Introduce the Hudson River Miles system; show students the Hudson River Miles map.
  4. Do "Tracking the Salt Front" worksheet in class; assign "Salt Front Math" as homework..

Vocabulary List and Answer Keys:

Available in the pdf version of this teacher's section and in the package that bundles all of the readings.


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


The U.S. Geological Survey Hudson River Salt Front website has historical data on the salt front's location plus real-time data on water temperature, tide stage, and other parameters from Poughkeepsie and Albany. The Hudson River Environmental Conditions Observing System [HRECOS] measures salinity and other water quality and weather parameters at sites from New York City to Albany and uploads this data to the web. On the HRECOS website, click on the Current Conditions page to access this information. Dropdown menus allow users to select a station and parameter, choose units of measurement, plot continuos readings (usually generated every 15 minutes) or daily averages, and specify start and end dates. One can also compare parameters by plotting two on one graph. Use the links leaving DEC's website on the right side of this page to visit the Hudson River Salt Front and HRECOS websites.