Exploratour - Water on Mars
Map of Valleys and Networks
This map shows a global distribution of outflow valleys ( in red) and river networks (in yellow) . The water features seem to be concentrated around Tharsis Ridge and the boundary between the highlands and the lowlands. The outflow channels seem to be found mainly on the slopes of the Tharsis Ridge.
This means that water bursted from reservoirs in the ground near the volcanoes on the Tharsis Ridge.