THE ANSWER: Lakes are fed by rivers, which in turn are fed by rainwater. As rainwater passes through soil and around rocks, it dissolves some minerals, including salt, but contains these minerals in very low concentrations. However, while lakes are fed by rivers, they are also drained by them.

Lake water is generally not salty because it is part of an active, flowing cycle that prevents minerals from accumulating. Most lakes are "open systems," meaning they are constantly fed by freshwater sources—like rain, melting snow, and rivers—and have an outlet (a river or stream) that carries water and dissolved minerals out toward the ocean. Because the water in a lake is "flushed" out and replaced frequently, the tiny amounts of salt washed from the surrounding rocks never reach a concentration that we can taste. In contrast, the ocean is the "final destination" for these minerals; when ocean water evaporates, it leaves the salt behind, but the salt has nowhere else to go, so it builds up over millions of years. However, if a lake is "closed" (like the Great Salt Lake or the Dead Sea) and has no outlet, the water can only leave through evaporation. In these rare cases, the salt stays behind and accumulates, making the lake even saltier than the ocean—a fascinating exception to the "freshwater" rule.