Causes

Salts originate in the lithosphere by weathering and other natural processes, and cycle between the biosphere and hydrosphere.

Among the various salts, the supply of sodium chloride far exceeds its demand by the biosphere.

The biosphere wastes excess salts.

Streams and rivers carry this excess salt to the oceans, where they accumulate [in geologic time].

In irrigation, human societies borrow the water to grow crops.

Fresh water has a small salt content [usually 300 ppm].

The salt is not used by the agricultural system, and it is wasted again back to the hydrosphere.

Only that this time, the water was consumed [used in crop evapotranspiration] but the salt was returned unused in the agricultural wastewaters [Example: Salton Sea].

So the salt concentration downstream [in streams, rivers, and lakes] tends to increase.

This is the problem of anthropogenic salinization of rivers.