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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.

Objectives:

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

Skills:

  • 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.

Duration:

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

Materials:

Each student should have:

Background:

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). Its location depends on freshwater runoff from the watershed. After heavy rains, runoff pushes the salt front seaward. As runoff lessens in 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. Tidal currents have minor impacts on salt front location, moving it a few miles up or down the river on each tide cycle. 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).

Activity:

  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.

Assessment:

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

Resources:

The U.S. Geological Survey Hudson River Salt Front website has tables and a map showing the front's location plus real-time data on water temperature, tide stage, and conductivity (a salinity indicator) from Hastings on Hudson, West Point, Poughkeepsie, and Albany (no conductivity data here). Use the offsite links on the right side of this page to visit the U.S. Geological Survey Hudson River Salt Front website.