Morphometric measurements of martian valley networks using Mars Orbiter Laser Altimeter (MOLA) topographic data yield mean valley width values of x¿¿&sgr;:2040¿1548 m and depth values of x¿¿&sgr;:109¿137 m. Our analysis of changes in valley shape with latitude and wall slope with depth indicate infilling and mass wasting processes have not greatly altered the original valley shape; thus valley network morphometry is dominantly due to channel formation mechanisms. Both U-and V-shaped profiles are observed, commonly within the same valley network system. The average U-shaped valley is slightly deeper (18 m) and significantly wider (1313 m) than its V-shaped counterpart. Valley networks have decreasing width-to-depth ratios and increasing wall slopes as valley depth increases. Our observations are consistent with a two-phase valley network formation model. (1) Valleys initially form via surface runoff, a process that creates V-shaped profiles and quasi-dendritic form. The observed linear correlation of top width and depth (below 125 m depth) suggests the depth of vertical incisement governs top width. (2) Reactivation of the same valleys by headward extending sapping processes widens the channel to form U-shaped cross sections in downstream reaches. The availability of liquid water within a few hundred meters of the surface appears to be a necessary requirement for valley network formation. Mean valley depth decreases by ~50 m from equatorial to higher latitudes (~50¿), contrary to the relationship predicted if the sapping depth was governed by the ice-water boundary. Deeper equatorial valley networks may result from latitudinal variations in the availability of water or formation efficiency. ¿ 2001 American Geophysical Union