Among of the better stories of Asimov.
Political problems between a human colony on Mars & administration of Earth is threatening to become a water crisis for colony - as earth is likely to refuse to supply them water!!
But colonists are a step ahead - they've moved an iceberg from one of the Saturnian rings to Mars to meet their water needs; no more water dependence on earth! In fact, they're about to offer supply of freshwater to earth a few years' hence!
Fact sheet.First published: Galaxy, November 1952.