The rounded pebbles found by the team from Nasa's Curiosity rover mission are only formed when they are carried through water over long distances, research published in the journal Science says.
It is the first time evidence of sustained water flow on Mars has been uncovered.
The report authors noted that conditions on Mars at the time the pebbles were deposited two billion years ago were vastly different to the cold, dry environ- ment of today if water was to flow over several kilometres.
However, they said the discovery of the fluvial deposits raised prospects for the former presence of habitable environments on Mars.
However, a separate study published in Science magazine, showed astronauts heading to Mars face a risk of exposure to high doses of radiation.