From noaa.gov's Ocean facts:
Salt in the ocean comes from rocks on land.
The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid (which forms from carbon dioxide and water).
But...if this is the case wouldn't our rivers be even saltier from seasonal downpours and snowpack melt?
In terms of concentration, I would say no. With higher rainfall and snowpack melt, you will have more solute and therefore a greater ability to dissolve saline species. However, you also have more water so the concentration of salts in the rivers shouldn't show a dramatic change (unless the river begins to run through a salt flat or something like that).
To make the rivers more saline, you would need to change other factors such as temperature, pressure, and input/output from the system. Since we're working with a river, pressure remains relatively constant at atmospheric conditions. For the relevant temperatures (0 C - 30 C), the solubility of NaCl in water changes from 35.65 g NaCl/100 mL water to 36.09 g NaCl/100 mL water. That's a ~1.2% change, which means temperature does not have a large effect over that range. This leaves us with the input and output from the system. Without an outlet, the water level will fluctuate between the effects of input and evaporation. Meanwhile, the salinity will only increase as more salt species are dissolved and introduced. Therefore, as long as the river keeps moving, its salinity will not change significantly.
No, because fresh rainfall has no salt in it, so as long as the river keeps moving the salt has no time to accumulate.
Only stagnant sources of water on land, like lakes with no outlet, can become saline.
