What evidence did Curiosity find that Gale crater once contained an ancient stream?

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Mars An especially serene view of Mars (Tharsis side), a composite of images taken by the Mars Global Surveyor spacecraft in April 1999. The northern polar cap and encircling dark dune field of Vastitas Borealis are visible at the top of the globe. White water-ice clouds surround the most prominent volcanic peaks, including Olympus Mons near the western limb, Alba Patera to its northeast, and the line of Tharsis volcanoes to the southeast. East of the Tharsis rise can be seen the enormous near-equatorial gash that marks the canyon system Valles Marineris.

In September 2012, the Curiosity rover provided evidence that Gale crater once contained an ancient stream by capturing photographs of water-transported gravel. This discovery supported the conclusion that the crater floor was likely shaped by flowing water in the planet's past.1