Kasei Valles is one of the largest outflow channel systems on Mars that was created by several gigantic flood events, brought about by the changing tectonic and volcanic activity in the nearby Tharsis region over 3 billion years ago.
Video: "Flight over Kasei Valles" from 2014 [4:43 min]
Copyright: ESA/DLR/FU Berlin
The source of Kasei Valles is Echus Chasma (south of the region shown in the movie) and they discharged into Chryse Planitia. Altogether the outflow channels cover a distance of 3000 km, of which the upper half is shown in the movie. On the floor of the channels volcanic as well as glacial deposits are found. Tectonic and volcanic activity in the nearby Tharsis region probably caused the discharge of such a large amount of water.
Link to press release: Kasei Valles Movie (Jan 14, 2014)