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How can diverting river water for irrigation allow saltwater to move farther upstream?

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Large wheeled irrigation sprinkler sprays water over green crops in a field, with mountains and cloudy sky in the background. — Irrigation Specialized irrigation equipment watering crops in a field.

Diverting river water for irrigation reduces the volume of water flowing downstream, which lowers the river's overall water level.1

In coastal areas, rivers often form estuaries, where fresh water meets and mixes with seawater. These systems are often characterized by a "salt wedge," a layer of denser salt water that pushes inland along the riverbed beneath the fresher surface water. The extent to which this salt wedge penetrates upstream is heavily influenced by the strength of the river's discharge; high river flows act as a barrier that pushes the salt water back toward the sea.23

When irrigation diversions significantly decrease the river's flow, this natural barrier is weakened. With less fresh water pushing seaward, the salt wedge is able to migrate farther upstream. This process, known as saltwater intrusion, can contaminate freshwater supplies and negatively impact agricultural land by increasing soil salinity.214