This complex outflow-channel system on Mars is up to 40 km wide in places. The northern Dao Vallis is about 2400 m deep, the southern Niger Vallis about 1400 m.
Video: "Flight over Dao and Niger Valles" from 2012 [3:17 min]
Copyright: ESA/DLR/FU Berlin (G. Neukum)
The animation shows the desiccated river channels Dao and Niger Valles (see also mosaic press release). It is assumed that water flowed through 3.5 billion years ago. The origin of the channels is still obscure. Maybe they were fed by surface run-off, like on Earth, or alternatively, were formed by ascending groundwater from subsurface sources. Another hypothesis claims that the channels were carved by water from melted ice.
Link to release in press archive: Dao/Niger Valles Animation (Feb 17, 2012)