Some morphometric differences between terrestrial and Martian valley networks may reflect the precursor topography on Mars, particularly impact basins or tectonic slopes. To evaluate these possible influences, we mapped highland watersheds in nine study areas that sample a range of geographic and topographic settings. We collected data including latitude, longitude, watershed length, divide and terminal elevations, watershed relief and slope, slope orientation, and qualitative descriptors including whether a drainage basin was open or closed. The longest valley networks and most overflowed basins occur on preexisting intercrater slopes of 0.1-1 degrees, particularly on north facing slopes associated with the crustal dichotomy. The control of watershed length by earlier Noachian topographic features, which the relict networks did not significantly modify, suggests that the Early to Middle Noachian geomorphic environment was nominally much drier than the later Noachian to Hesperian transition. The distribution of fluvial valleys and likely orographic effects created by the crustal dichotomy suggest that evaporation from the northern lowlands was an important source of atmospheric humidity over short time scales. Much of the highland plateau consists of smaller enclosed watersheds, which (along with cooler temperatures) detained surface water at high elevations, lengthening or impeding the global water cycle. Ponding and evaporation may have partly offset a continentality effect of the highland landmass. Prolonged modification of the intercrater geomorphic surface prior to incision of valley networks included substantial weathering, reduction of relief, and gravity-driven sediment transport, indicating a long-term role for surface water in a transport-limited, arid to hyperarid Noachian paleoclimate.
