Ares Vallis is a Martian outflow channel which is about 1500 km long and locally exceeds 100 km in width and 1000 m in depth. We estimate the channel's paleoflow capacity using paleohydraulic techniques. At a constricted and unusually deep reach, not affected by tributaries or secondary inflows, the estimated possible maximum peak discharges are of the order of 108-109 m3/s. These values are 1 to 2 orders of magnitude larger than any known terrestrial floods. These high-discharge flows were theoretically capable of transporting boulders larger than 10 m in diameter. The downstream depositional plain, where Ares debauches into the Chryse Planitia, displays geomorphological features indicative of high erosional capacity by the flooding. In this area, site of the proposed Pathfinder landing, we expect to find a complex sequence of sediments created by varying paleoflood flow hydraulics, sediment transport characteristics, and probable secondary modification of the primary flood landforms.¿ 1997 American Geophysical Union