Rivers play an important role in our ecosystem. They provide water for drinking and irrigation of crops, a habitat for fish and other organisms, and routes to easily transport goods. For these reasons and more, it is important to monitor the quality of river water, including its physical, chemical, and biological characteristics.
One often measured parameter is specific conductance. Conductance is a measurement of a substance’s ability to conduct electricity and is related to the amount of ions, like salt, that are dissolved in the water.
Where rivers meet the ocean, the salt typically comes from seawater flowing upstream into the river. How far upriver the saltwater can reach (often called the salt front or salt line) depends greatly on an estuary’s type and the current streamflow.
Further upstream, where the ocean doesn’t have as much influence, the amount of dissolved salts in river water is generally related to how much precipitation there has been. When rainfall is light, more water on land can evaporate before it reaches the river, which concentrates the amount of dissolved salts in the water that remains. When rainfall is heavy, water tends to flow more quickly into rivers and streams, with smaller concentrations of dissolved salts.
The image above shows the relationship between river flow (discharge) and conductance over a 3+ year period on the Delaware River in Trenton, NJ. In general, the conductance is quite low, and well below accepted salt front cutoffs of ~400-1,000 micro-Siemens per centimeter, which correspond to chloride concentrations of 100-250 milligrams/liter. However, there is clearly an inverse relationship between conductance and discharge. When discharge is strong, water conductance is low, though it never gets below ~100 µS/cm. Likewise, when discharge is light, conductance is 2-3 times higher. While the salt concentration on the Delaware River near Trenton is generally low, it does depend on the streamflow.
Knowing the location of the salt front is important, especially on rivers where water is drawn for human consumption or irrigation, and for protecting riverine infrastructure like ships that corrode more easily in salty water. The Delaware River Basin Commission regularly monitors the salt front location in order to control its location by storing and releasing water in reservoirs upstream.