Department of Environmental Conservation

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Finding the Salt Front

Lesson Plan

Students will use Hudson River salinity data to create a line graph that shows the location of the salt front, and use math skills to explore how this location varies over time.


Students will use data from tables to:

  • graph salinity data from sites along the Hudson River estuary;
  • observe patterns of change in salinity along the estuary;
  • use the graph to estimate the location of the salt front;
  • compare the location of the salt front in different years.

Grade level:

Elementary (Grades 4-7)

Subject Area:

Math, Science


Mathematics, Science, & Technology Standards 3, 4


  • Use graphs to see patterns and relationships observed in the physical environment.
  • Use whole numbers to identify locations and measure distances.
  • Add and subtract whole numbers.


Preparation time: 5 minutes
Activity time: 40 minutes for each of two sections


Each student should have:


Tidal from New York Harbor to Troy, the lower Hudson River is an estuary where fresh water and salty seawater meet. Fresh water dilutes the seawater entering the Hudson; its leading edge, called the salt front, is where the concentration of chlorides (sodium chloride-table salt-is an example) reaches 100 milligrams per liter (mg/L). Low concentrations of salt (20-50 mg/L) are found in fresh water north of the salt front, due to erosion and human activity.

Salinity greatly influences where the estuary's animals and plants are found. Some live only in fresh water, others only in salt. A few, like the blue crab, can survive in fresh or salt water.

The salt front's position depends on runoff from the watershed, which varies with seasonal climate patterns and weather events. Scientists give its location using Hudson River Miles. Hudson River Mile (HRM) 0 is at the Battery at the southern tip of Manhattan. The estuary part of the Hudson ends at the Federal Dam in Troy at HRM 153


  1. Review the terms estuary, salinity, and salt front, and ask how salinity might influence where animals and plants live.
  2. Explain Hudson River Miles and how upriver and downriver relate to north and south.
  3. Do section 1 of worksheet in class; assign section 2 as homework.
  4. Follow up with Which Fish Where? lesson on how salinity influences fish distribution.


  • Have students share answers to questions from worksheets, or collect and grade sheets.
  • Make up similar problems for quiz. Have Students define the salt front in thier own words.

Resources: (see Links Leaving DEC's Website in right-hand column)

The U.S. Geological Survey's Hudson River Salt Front website has tables of historical data showing the salt front's location over time. The site also displays real-time data for 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 continous 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.

On DEC's annual Day in the Life of the Hudson River, thousands of students and teachers collect data at field sites from New York Harbor north to Albany and beyond. Their results are posted on the Day in the Life (a.k.a. Snapshot Day) website, which supplied data for this lesson. Note that salinity is measured in various ways, and some data had to be converted to equivalent mg/L of chloride.