A lot of rivers. We put a photo recon satellite, the Mars Global Surveyor, into orbit around Mars in 1997. It carried wonderful cameras that returned zillions of sharp clear photographs of the Martian surface. The best of the pictures are collected in a softback book "A travelers guide to Mars", William K. Hartmann. Thumbing thru this book, the dried up river beds are striking, and there are lot of 'em. There is no question that they are rivers, even to my layman's eye they really look like rivers. You can see deltas at the end of them where they flowed into ancient Martian seas. Shades of Edgar Rice Burroughs
Some of them flowed recently (like within the last 10 million years). You can tell by counting meteor craters. Old (going back to the formation of Mars) land is wall to wall craters. New land, recent lava flows, has fewer craters, partly because they have had less time to accumulate meteor hits, and partly because the meteor hit rate has dropped off in more recent times. The plentiful meteors at the time of solar system formation got swept up by planets over time. Some of the rivers have no craters at all, which makes them very recent.
The unanswered question is, where did the water come from, and where did it go? We have found a few dozen meteorites on Earth that we believe came from Mars. Some have been dated back to 4500 megayears (pretty much the formation of Mars) and some to as recently as 167 megayears. All of them had been soaked in liquid water at some time in their past, as evidenced by deposits of water borne minerals in cracks and crevices. So there was a lot of water on Mars, as recently as the youngest meteorites. We think the water is still there, soaked into the soil and frozen.
We think Mars has been cold, and short on atmosphere, as it is today, for the last 3000 megayears. So how did the water to form river beds as recently as 10 megayears ago come from? No good answer has been proposed as of yet. We think there is plenty of water frozen in the Martian soil, but how did it melt and flow on the surface? No one knows.
We now think that Mars had open water, seas and rivers from the beginning, say 4500 megayears ago, until perhaps 3000 megayears ago. That gives 1500 megayears for some kind of life to evolve in Martian seas. Perhaps some such life still exists somewhere on Mars.