Which rover discovered evidence of ancient stream beds on Mars?

Curiosity discovered evidence of ancient stream beds on Mars. In September 2012, shortly after landing in Gale crater, the rover examined rounded pebbles and rock deposits that appeared to have been shaped and transported by flowing water.

Scientists identified the feature in an area called Hottah, near Curiosity's landing site. The stones were too rounded and tightly packed to be explained easily by wind alone, and their shapes indicated movement in a once-active stream. The evidence showed that water had flowed vigorously across parts of Gale crater billions of years ago.

This discovery was important because it established that ancient Mars once had environments capable of sustaining surface water. Curiosity's broader mission is not to find modern life directly, but to determine whether Mars ever had conditions suitable for microbial life. Later investigations of Gale crater's rocks and Mount Sharp added evidence of ancient lakes, rivers, and changing environmental conditions. Perseverance studies a different ancient water environment in Jezero crater.