Distribution of drainage systems on Mars. The yellow corresponds to fluvial valley networks and the red to outflow channels. The drainage systems on Mars do not follow a distribution in watersheds or latitude, but are widely spreaded at the surface of the most ancient cratered highland. Some systems are observed on younger terrain, but they are less numerous. Drainage densities are quite weak on Mars (number of kilometers drained drained by square kimlometer), values matching at best those of terrestrial arid regions. Figure adapted from Carr.
Source: NASA