Fresh and salt water systems
Almost all of Earth's water is salt water, and the small fresh share moves through watersheds, rivers and the ground.
A worked example
A creek carrying 1.8 cubic metres of water per second delivers 1.8 x 60 = 108 cubic metres every minute.
The mistake people actually make
A watershed is set by the high ground around it, not by town or property lines, so something spilled anywhere in it can reach the same river. Fresh water is scarce in terms of what is reachable and clean, not only what exists.