The Viking Orbiters caused a revolution in our ideas about water on Mars; finding huge river valleys in many areas.
Space craft cameras showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers.
On the other hand, some of the large outflow channels begin in rubble-filled low areas, called chaos or chaotic terrain.
Lineated floor deposits may be related to lobate debris aprons, which have been proven to contain large amounts of ice.
[9] Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hot magma melted huge amounts of ice in the frozen ground.
The partially circular depressions on the left side of the channel in the image below suggests that groundwater sapping also contributed water.