There is enormous evidence that water once flowed in river valleys on Mars.
Images of curved channels have been seen in images from Mars spacecraft dating back to the early seventies with the Mariner 9 orbiter.

Vallis (plural valles) is the Latin word for valley.
It is used in planetary geology for the naming of landform features on other planets, including what could be old river valleys that were discovered on Mars, when probes were first sent to Mars.
The Viking Orbiters caused a revolution in our ideas about water on Mars; huge river valleys were found 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.
Some valles on Mars (Mangala Vallis, Athabasca Vallis, Granicus Vallis, and Tinjar Valles) clearly begin at graben.
On the other hand, some of the large outflow channels begin in rubble-filled low areas called chaos or chaotic terrain.
It has been suggested that massive amounts of water were trapped under pressure beneath a thick cryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.
There is enormous evidence that water once flowed in river valleys on Mars. Images of curved channels have been seen in images from Mars spacecraft dating back to the early seventies with the Mariner 9 orbiter. 
Vallis (plural valles) is the Latin word for valley. It is used in planetary geology for the naming of landform features on other planets, including what could be old river valleys that were discovered on Mars, when probes were first sent to Mars. The Viking Orbiters caused a revolution in our ideas about water on Mars; huge river valleys were found 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. Some valles on Mars (Mangala Vallis, Athabasca Vallis, Granicus Vallis, and Tinjar Valles) clearly begin at graben. On the other hand, some of the large outflow channels begin in rubble-filled low areas called chaos or chaotic terrain. It has been suggested that massive amounts of water were trapped under pressure beneath a thick cryosphere (layer of frozen ground), then the water was suddenly released, perhaps when the cryosphere was broken by a fault.
Iapygia quadrangle