Why are the oceans/seas salty, but rivers and lakes remain fresh water?
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. The lakes and rivers stay fresh because not much salt is picked up in rivers and all the salt stays in the ocean, and the lakes aren’t salty because there is a low concentration of salt, so to get a salty lake you’d need a lake about 10 times as big as lake Superior or a high concentration of salt.
