I'm curious to find out why we talk of freshwater (or fresh water) when we refer to water with a very limited amount of salt dissolved in it.
Looking at various sources, both online and in books, I have learnt that the term sweetwater may also be used.
Do you know the origin of the expression and is sweetwater a valid alternative?
The NOAD identifies the etymology of fresh:
ORIGIN Old English fersc [not salt, fit for drinking,] superseded in Middle English by forms from Old French freis, fresche; both ultimately of Germanic origin and related to Dutch vers and German frisch.
Sweetwater, I would assume, is at this point (don't know about originally; cf. @BarrieEngland's answer) the antonym of saltwater, and therefore synonymous with freshwater.
The OED’s earliest citation for ‘fresh water’ is 1528, in the phrase ‘the best freshe water fyshe’. ‘Sweet water’ appears around the middle of the sixteenth century, in the sense of ‘a sweet-smelling liquid preparation’. It is first used to mean fresh water in 1608.
My contention has always been that:
Rain water reaches the ground, is partly utilized, and eventually flows into the sea.
Therefore, water freshly obtained from rain, from water-bodies or ground water is fresh and potable, unlike that which has washed out into the sea to become salt water heavy with minerals.
