After years and years of river inflow and evaporation, the salt content of the lake water built up to the present levels. The same process made the seas salty. Rivers carry dissolved salts to the ocean. Water evaporates from the oceans to fall again as rain and to feed the rivers, but the salts remain in the ocean.
Simply saying, main source of salts is dissolved salts of salty rocks in the rivers' water 
you may say that the sea water has much salts than rivers water, the answer is that due to evaporation of sea water its concentration become higher than rivers 
so concentration of salt of the sea water is a function of the amount of entering fresh water of rivers (fresh water means the water containing less salt) and the amount of evaporation of the water
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