Mars Climate, Now and In the Past
Valles Marineris, the grand valley of Mars named after the Mariner program which first took close-up images.
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Image from: USGS
Click on image for full size
Image from: USGS
For more evidence, take the Exploratour of the Water on Mars.
Running Water, Contd.
This picture of the canyons of Valles Marineris shows some of the evidence for running water. Scientists have found lots of evidence like this.