Martian outflow channels and valley networks raise six questions and the first four are addressed in this part: 1) how were outflow channels and valley networks carved, likely from water?; 2) Were the channels and valley networks eroded by a few catastrophic floods or many small flows?; 3) What is the origin of the water: groundwater, rainfall and runoff, or both? If groundwater, how was it recharged?; and 4) What is the quantity of water needed for erosion; estimates varying from 3 m to 5000 m GEL. Ultimately, did water come from impacts, volcanism, or both?