The Hudson River is an estuary: a freshwater river that flows into the ocean, where it mixes with salt water. The degree of mixture between fresh and salt water is called the salinity gradient. On the Hudson, the salinity gradient at a given location can vary considerably depending on factors such as time of year, tidal flow, and precipitation. Salinity levels have an immediate effect on the surrounding ecology, since many aquatic creatures can survive only in either "salt" or "fresh" water. It's just as important for sailors (of any era), who need a fresh water supply to survive.
To measure the salinity gradient as we travel upriver, our students collect water samples at a series of predetermined locations, which our students find using landmarks (such as buoys and river markers) and GPS coordinates.