The cratered highlands of Mars have many river valleys that formed in a water-rich environment about 4.5 to 3.7 billion years ago. Erosion since then has been slow, leaving these very old features preserved. Later, massive outflows of groundwater formed flood channels tens to hundreds of kilometers wide and perhaps over 1,000 kilometers (620 miles) long.
River valley networks begin near crater rims and other high ridges and wind through the cratered landscape. Valleys in this area drained into impact craters, some of which appear to have filled with water and overflowed, forming additional deep valleys (shown by arrows) to the north.
Mars Odyssey, THEMIS; NASA/JPL/Arizona State University image
Mars Orbiter, Laser Altimeter; NASA/JPL/GSFC image