Water is water, but why are there such drastic differences between the composition of ocean water and water flowing in rivers and lakes? The easiest way to come to a conclusion for this question is to follow the cycle of water. Water falls to earth from clouds and in its purest form, simply water fused with carbon dioxide gas. As it lands, it dissolves minerals and salts (sodium, chloride, sulfate, magnesium, calcium, potassium, bicarbonate, and bromide) from the ground and makes its way towards rivers and lakes, carrying the aforementioned minerals and salts with it.
The rivers and lakes ultimately flow into the ocean, becoming saltier as they get closer to their destination because more minerals are absorbed en route. When a river meets the ocean, it forms an estuary. Estuaries are comprised of brackish water, water that is not as salty as the ocean but has more salts and minerals than a river. Finally, after its long trek, the water reaches the ocean. From the ocean, the water has only one way out, evaporation. As the sun beats down on the ocean the water evaporates but salts and minerals are unable to evaporate and remain in the ocean.
The water is then transported by clouds, falls as precipitation and cycles through the process again depositing more salts and minerals into the ocean. As time goes by, more and more minerals and salts are deposited in the ocean causing it to get saltier with each passing year. Since the minerals and salts in rivers flow out into the ocean with the water, rivers do not accumulate salts and minerals.
Lakes are a middle ground, whereas some have rivers that connect them to oceans allowing the salts and minerals a way out, while others are landlocked and the water has no other way out except evaporation. The Great Salt Lake and the Dead Sea are two examples of landlocked lakes that have a high level of salinity. Occasionally a landlocked lake will completely dry up leaving a salt flat in its place until the next rain restores the lake and absorbs the awaiting salts and minerals.
The simplest answer to the query “why are oceans salty and lakes and rivers are not” would be that while water can evaporate out of the ocean salts and minerals are too heavy to and are imprisoned in the ocean and landlocked lakes without an escape route. Rivers, while containing salts and minerals, deposit them into oceans allowing the minerals a way out so they do not build up their salinity.