Summary

Breaches of large natural basins, usually initiated by high runoff or meltwater production in their contributing watersheds, have been responsible for the most intense recognised terrestrial floods. Some of the many impact craters and intercrater basins in the Martian highlands also apparently overflowed during the Noachian Period (>3.7 Ga), forming relatively wide and deep outlet valleys. Broad, mid-latitude basins overflowed to carve Ma'adim Vallis and the Uzboi–Ladon–Morava Valles system, which are similar in scale to the terrestrial Grand Canyon but record much larger formative discharges. Other valley network stems of comparable size are also associated with smaller breached basins or broad areas of topographic convergence, and even the smaller basin outlets are typically deeper than other valleys in their vicinity. Little evidence for catastrophic (by terrestrial standards) meteorological floods has been recognised to date in Martian alluvial deposits. For these reasons, basin overflows may have been disproportionately important mechanisms for valley incision on Mars. Many of the Martian outflow channels also head in topographic settings that favoured ponding, including large canyons, impact or intercrater basins, chaotic terrain basins and grabens. Draining of this topography may have supported peak discharges of ∼106–108 m3 s−1, particularly in the largest channels, but the basin overflow mechanism does not eliminate fully the need for large subsurface outflows.