They did not come from the ocean, they came from the precipitation of clouds which is fresh water. Later they will eventually reach the ocean. 
Most of the time, they are not exactly fresh due to the surroundings. The reason why they can sustain the area that they do though, is primarily because of snow in the mountains which runs down to the rivers and lakes which we have. Another factor for this is Rain, which is caused from evaporation of h20 which then comes down as rain. Even then, rain is polluted with all of the pollutants in the air.this will surely help u i know
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