A large, ancient river valley, called Ma'adim Vallis, enters at the south rim of Gusev Crater, so Gusev Crater was believed to be an ancient lake bed.
 However, it seems that a volcanic flow covered up the lakebed sediments.
Apollinaris Patera, a large volcano, lies directly north of Gusev Crater.

Recent studies lead scientists to believe that the water that formed Ma'adim Vallis originated in a complex of lakes.
The largest lake is located at the source of the Ma'adim Vallis outflow channel and extends into Eridania quadrangle and the Phaethontis quadrangle.
When the largest lake spilled over the low point in its boundary, a torrential flood would have moved north, carving the sinuous Ma'adim Vallis.
At the north end of Ma'adim Vallis, the flood waters would have run into Gusev Crater.

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.
A large, ancient river valley, called Ma'adim Vallis, enters at the south rim of Gusev Crater, so Gusev Crater was believed to be an ancient lake bed.  However, it seems that a volcanic flow covered up the lakebed sediments. Apollinaris Patera, a large volcano, lies directly north of Gusev Crater.
Recent studies lead scientists to believe that the water that formed Ma'adim Vallis originated in a complex of lakes. The largest lake is located at the source of the Ma'adim Vallis outflow channel and extends into Eridania quadrangle and the Phaethontis quadrangle. When the largest lake spilled over the low point in its boundary, a torrential flood would have moved north, carving the sinuous Ma'adim Vallis. At the north end of Ma'adim Vallis, the flood waters would have run into Gusev Crater.
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.
Aeolis quadrangle