Large flood channels, termed outflow channels, are observed incised into the Martian surface in several areas. The channels are much larger than the valley networks, generally being tens of kilometres across and hundreds of kilometres long. Most emerge full-sized from rubble-filled depressions and continue downslope into the northern plains or the Hellas basin in the south. Many of the largest drain from the south and west into Chryse Planitia. These are true channels in that they were once completely filled with flowing water, as opposed to most river valleys, which have never been close
Three outflow channels located near the eastern edge of the giant impact basin Hellas, in a view obtained by Mars Global Surveyor on September 13, 2000. The channels are roughly 1 km (0.6 mile) deep and vary along their courses from about 40 km (25 miles) to about 8 km (5 miles) in width. NASA/JPL/Malin Space Science Systems to full but contain a much smaller river channel. The peak discharges of the floods that cut the larger outflow channels are estimated to have been a hundred to a thousand times the peak discharge of the Mississippi River—truly enormous events. Some of the floods appear to have formed by catastrophic release of water from lakes. Others formed by explosive eruptions of groundwater. The outflow channels are younger than the valley networks and probably mostly formed when conditions were similar to those that prevail today. Recent discovery of very young outflow channels suggests that they could form today by eruption of groundwater from below the kilometre-thick permanently frozen ground.
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