Northern Arabia Terra, Mars is characterized by a complex landscape that includes fretted terrain, fretted channels, completely closed canyons, shallow troughs, and fracture-bounded depressions. The importance of ground ice in the initiation and development of these features, and of large impact structures in controlling channel courses, has long been recognized. The development of the fretted channel systems, including the integration of closed canyons, craters, and surface depressions into throughgoing channels, has previously been attributed primarily to mass-wasting processes. The two channel systems in southern Ismenius Lacus quadrangle, Mamers Valles and Ismeniae Fossae, incorporate craters of probable impact origin, but both also include irregular crater-like features and scalloped channel reaches that have characteristics not consistent with an impact origin. Some of these are, by analogy with morphologically similar features on Earth and elsewhere on Mars, reasonably interpreted as of volcanic origin. The detailed morphologies of a few diagnostic features associated with Ismeniae Fossae suggest a hydrovolcanic origin, an inference supported by the probable presence of abundant ground ice or groundwater at the time of their formation. The evidence described here for hydrovolcanism supports the inference that, in addition to mass-wasting processes, explosive excavation, intrusion-driven sublimation, subsurface erosion, and erosion by running water may have played important roles in channel formation and evolution as well. All of these processes are consistent with magma interacting with groundwater or ground ice. ¿ 1998 American Geophysical Union