NASA's Mars rover, Curiosity, has determined that a river from ankle to knee deep once flowed on Mars for thousands or even millions of years. The next question is whether it ever supported life. Per the article:

Less than two months after touching down inside a giant impact basin near the planet's equator, Curiosity has returned clear evidence of flowing water, scientists told reporters during a conference call Thursday....Curiosity scientists are quite sure the gravel was deposited by a vigorously flowing stream, one that was between ankle and knee-deep and likely flowed for thousands or even millions of years.

http://news.discovery.com/space/mars-rover-finds-bed-of-ancient-flo...