Why is sea water salty, when all the rivers pour clean water in it? why can't we drink it?

1) First , although every river pours the clean water in sea , the river water always contains some minerals that may have dissolved from soil or some other reasons.

every time river water enters in sea , with seasonal changes , the water evaporates because of the solar radiations. In this method , only water molecules get evaporated but minerals & salts remain in sea water.Hence the sea water becomes more salty.

2) As far as our body is concerned , it can work faithfully only when the pH of our blood remains in particular range. when we drink any water which has relatively less or more pH , it get directly assimilated with blood tending change its pH . It is very fatal as in extreme cases , change in blood pH is a major cause of death . Hence it is not advisable to drink sea water/salty water continuously.