Erosional features on a plateau in the Valles Marineris provide evidence that a lake filled Capri Chasma until it overflowed its eastern rim, carving two large spillover channels. The floodwaters surged into the adjacent lowlands of ancestral Ganges Chasma and eastern Eos Chasma. The channel floor elevation and depth of incision at Daga Vallis reveal that a 1200-m-deep lake water column was drained by the flooding. The width, depth, and steep energy slope (∼0.02) of Daga Vallis and the existence of several cataracts, including a 500-m-high dry falls, demonstrate the power of the floodwaters. We estimate a possible peak discharge rate of 1–6 × 10⁸ m³·s⁻¹. The catastrophic flows may have been triggered by the collapse of topographic barriers in eastern Coprates Chasma, providing a gateway for lake waters in the central Valles Marineris to pour eastward toward Capri Chasma and the lowlands beyond. These may have been among the earliest flows in Simud-Tiu Valles, contributing discharge to a possible sea in the northern plains of Mars.