6 [1] The role of groundwater in forming Martian valley networks is simulated in a 7 computer model as seepage erosion by contributing to surface runoff and as seepage 8 weathering by causing accelerated weathering of bedrock, which makes its subsequent 9 erosion and removal easier. Simulation results show that seepage erosion cannot mobilize 10 large grain size sediment and is marginally effective at generating integrated valley 11 networks with realistic rates of aquifer recharge. On the other hand, seepage weathering 12 may play a major role in forming Martian valley networks. Seepage weathering combined 13 with fluvial runoff creates stubby deep canyons with abrupt headwalls that are similar 14 in morphology to terrestrial and Martian valley systems attributed to erosion by 15 groundwater. Depending on the relative contribution of groundwater weathering to surface 16 runoff erosion, a continuum of valley network morphology can be generated. Eolian 17 modification masks the original differences in fluvial landforms, making different 18 scenarios visually more similar. Martian valley networks may have developed through a 19 range of combinations of runoff erosion and seepage weathering, which can complicate 20 the interpretation of the processes based on final landform morphology. Unequivocal 21 identification of seepage involvement of valley incision on Mars may not be possible 22 without knowledge of subsurface properties (hydraulic conductivity, layering, degree of 23 cementation, etc.) and the grain sizes of sediment transported through the valley systems.