How!

[Adios]

As water flows in rivers, it picks up small amounts of mineral salts from the rocks and soil of the river beds. This very-slightly salty water flows into the oceans and seas. The water in the oceans only leaves by evaporating (and the freezing of polar ice), but the salt remains dissolved in the ocean - it does not evaporate. So the remaining water gets saltier and saltier as time passes.

SALINITY
Sea salts    Parts per Thousand
chloride    19.3 o/oo
sodium    10.7 o/oo
sulfate    2.7 o/oo
magnesium    1.3 o/oo
calcium    0.4 o/oo
potassium    0.4 o/oo
bicarbonate    0.15 o/oo
bromide    0.07 o/oo
other    0.06 o/oo
Total Salinity    35.08 o/oo
The salinity (salt content) of ocean water varies. The oceans and seas contain roughly 5 x 10 16 tons of salts. One cubic foot of average sea water contains 2.2 pounds of salt.

The oceans are about 3.5% salt (by weight). Salinity is generally reported in terms of parts per thousand (abbreviated o/oo), the number of pounds of salt per 1,000 pounds of water; the average ocean salinity is 35 o/oo.

The saltiest water is in the Red Sea and in the Persian Gulf, which have a salinity of about 40 o/oo (due to very high evaporation rates and low fresh water influx). The least salty seas are in the polar regions, where both melting polar ice and a lot of rain dilute the salinity.


But I can be serious too.

[Cainad (dec.)]

Does the ocean stay salty when billions of cubic feet of fresh water flow into it year after, decade after decade?

Much of the salts dissolved in the ocean comes from runoff from the continents. Freshwater rain erodes minerals that then flow into the oceans. That's how it got salty.

It stays salty for reasons already explained; the water evaporates off the oceans, the salt stays put.


How has this cycle stayed in equilibrim over billions of years (not really though because I'm pretty sure the average salinity of the oceans has changed quite a bit over the course of Earth's history)?

Probably Jesus, or Xenu.