While earlier evidence has been found intimating the one-time presence of water on Mars, this evidence, which consists of images of rocks containing ancient streambed gravels, is the first of its kind. In a statement, Curiosity science co-investigator William Dietrich of the University of California, Berkeley said:
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep. Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."
The location of the finding is between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater. While asserting the past presence of water, scientists are not yet able to say how long ago - or for what length of time - water flowed in the area.
Along with announcing their discovery, NASA also released images as evidence of the ancient stream's existence. You can see those images in a slideshow here.