It's been common knowledge for some time now that Mars was in fact covered in oceans and rivers similar to our home planet Earth....
Recently scientists discovered a new ancient river located in the upper part of the Reull Vallis region of Mars.
-Check out the 3D images below:
This river was huge. The image data from ESA's Mars Express spacecraft shows that, at some points, the riverbed is 4.3 miles (7 kilometers) wide and 984 feet (300 meters) deep. The stereo cameras onboard the satellite have also revealed "numerous tributaries" that fed the gigantic river.
The ESA Mars Express team says that the river was flowing with abundant water about 3.5 to 1.8 billion years ago, during the Hesperian period. After that, the Amazonian era started, causing the Reull Vallis to be invaded by a glacier. This glacier carved the valley where the river was, pushing debris and ice, and causing the sharp sides you see in these images.