Geomorphic mapping of a ~1 million square kilometer section of Terra Cimmeria on Mars (1:1M scale) indicates that prolonged, intense fluvial erosion occurred during the period of heavy bombardment. Crater counts date the termination of ubiquitous, intense erosion to the late Noachian, although some valleys may have continued downcutting into the early Hesperian. Stratigraphic and topographic relationships indicate that early erosional processes created large, integrated drainage basins, affected primarily by large impact basin structures and regional slopes. This terrain is not consistent with an origin purely by volcanic or impact processes. Cratering competed with drainage basin development, minimizing valley length, catchment area, and valley network integration. Drainage basin disruption resulted from impacts on valley thalwegs, or when the ejecta of large (usually >75 km) craters created low divides. Near-level upland intercrater plains are lightly dissected, in part because smaller craters could interrupt flow paths on gently sloping or flat terrain. Some closed drainage basins became integrated by continued erosion of drainage divides and stream capture, overflow of the divides, or headward growth of valleys fed by groundwater collected in closed basins. Drainage divide breaching was most effective on steeper (>0.5¿) regional slopes, where observed drainage density is also highest. This is due to greater runoff volumes and velocity encouraging valley incision, as well as steep slopes allowing valleys to bypass or breach superimposed craters. Valley systems commonly extend nearly to the crests of sharp drainage divides, and spatially ubiquitous valley source points throughout the higher elevations suggest that runoff derived largely from precipitation.