Nanedi Vallis, Mars - This valley several hundreds of kilometers long displays only few tributaries along its course. It is believed that its water supply came from the drainage of groundwater. On the other hand, the valley is deep, large and meandering, proofs that the valley was active for a long time. Wheter or not it was formed in one long episode or several flow episodes spaced in time (possibly ten to thousands of years apart) is still to be determined. The valley is about 2 km across. In the top portion of the image, a channel can be seen at bottom of the valley. The channel was carved by water. It reflects the width of the flow and the real dimension of the water conduit, whereas the valley represents the final product of both water erosion and valley erosion through mass wasting.
Source: NASA, JPL, Malin Space Science Systems