Eastward from here there is about a 0.03 degree upward slope before reaching the outflow channels, which means that if you filled the canyon with fluid, it would create a lake with a depth of 1 km (3,300 ft) before the fluid would overflow out onto the northern plains.

[2] Keith Harrison and Mary Chapman described strong evidence for a former lake in the eastern part of Valles Marineris, especially in Coprates Chasma.

Still, its volume of 110,000 cubic miles would be comparable to Earth’s Caspian and Black Seas.

Also, the low point in Eos Chasma where water would be expected to overflow is marked by fluvial features.

[3][4] The bottom of the Coprates Chasma contain a large field of small pitted cones which have been interpreted as Martian equivalents of terrestrial igneous[5] or mud volcanoes.