This Viking image of the head of Ravi Vallis show a 300-km long portion of a channel. Like many other channels that empty into the northern plains of Mars, Ravi Vallis originates in a region of collapsed and disrupted (chaotic) terrain in the cratered uplands. Structures in these channels suggest that they were carved by a fluid (most likely water) at high flow rates. The abrupt beginning of the channels, with no apparent tributaries, suggests that the water was released under great pressure from beneath a confining layer of frozen ground. As this water was released and flowed away, the overlying surface collapsed, producing the disruption and subsidence shown here. The flow in this channel was from west to east (left to right).
Source: NASA