For such dendritic valleys to form, running water had to have been present on Mars. The various sources of this water – precipitation, groundwater or glacial meltwater – can often be determined by examining the valley structure. Martian valley networks with a dendritic layout were most likely formed by surface runoff of precipitation or meltwater. The heads of the valleys are typically located on a topographical ridge such as a watershed, and the course of the runoff channels follows the local gradient. The term ‘watershed’ refers to the boundary between two adjacent river systems, which usually extend along ridges.
As can be deduced from the color-coded elevation model, the water flowed from north (on the right in the image) to south. The largest valleys in the images are up to two kilometers wide, and reach a depth of up to 200 meters. In particular, those that run in an east-west direction show heavily eroded valley edges, scoured by the erosive power of the water flowing down the valley.