As we know, Opportunity and Spirit saw lots of evidence for water, largely in their exploration of minerals inside rocks. But with this image, scientists have the first observation of flowing water’s influence. Right after Curiosity landed and sent its first pictures back, scientists thought they may have put down in an alluvial fan--an ancient runoff site, and a bullseye for the type of science Curiosity is designed to do.
Very early in its mission, Curiosity found this gravel, which was once part of an ancient stream bed. This image shows the Martian rock outcrop where that gravel is found.