outflow channel

Outflow channels on Mars are large geological features, much wider and longer than typical river valleys, that are incised into the Martian surface. These channels are believed to have been formed by massive, catastrophic floods of water, with estimated peak discharges hundreds to a thousand times that of the Mississippi River. Some of these floods may have originated from the sudden release of water from lakes, while others could have been caused by explosive eruptions of groundwater.1

The channels are generally observed emerging full-sized from depressions and flowing downslope into the northern plains or the Hellas basin in the south. Recent discoveries suggest that younger outflow channels may still form today through the eruption of groundwater from beneath the planet's permanently frozen ground. These channels are distinct from the Martian valley networks, which are thought to have formed over a longer span of Martian history, likely with water as the primary erosive fluid.12