A complete digital elevation and bathymetry model of Taiwan provides the opportunity to characterize the topography of an emerging mountain belt. The orogen appears to form a continuous wedge of constant slope extending from the subaerial peaks to the submarine basin. We compare submarine channel systems from the east coast of Taiwan with their subaerial counterparts and document a number of fundamental similarities between the two environments. The submarine channel systems form a dendritic network with distinct hillslopes and channels. There is minimal sediment input from the subaerial landscape and sea level changes are insignificant, suggesting that the submarine topography is sculpted by offshore processes alone. We implement a range of geomorphic criteria, widely applied to subaerial digital elevation models, and explore the erosional processes responsible for sculpting the submarine and subaerial environments. The headwaters of the submarine channels have steep, straight slopes and a low slope-area scaling exponent, reminiscent of subaerial headwaters that are dominated by bedrock landslides. The main trunk streams offshore have concave-up longitudinal profiles, extensive knickpoints, and a slope-area scaling exponent similar in form to the onshore fluvial domain. We compare the driving mechanisms of the likely offshore erosional processes, primarily debris flows and turbidity currents, with subaerial fluvial incision. The results have important implications for reading the geomorphic signals of the submarine and subaerial landscapes, for understanding the links between the onshore and offshore environments, and, more widely, for focusing the future research of the submarine slope.