We document and characterize hanging valleys in a fluvially eroded landscape in eastern Taiwan. Our conceptual model for the initiation of hanging valleys builds on a recently proposed model of bedrock incision in which erosion actually becomes less efficient on very steep channel gradients. If a pulse of incision in the main stem outpaces the tributary response, the gradients at tributary mouths may therefore pass a threshold value beyond which erosional efficiency declines, giving rise to a mismatch between trunk and tributary erosion rates. This mismatch is expected at junctions with small tributaries, where a step function decrease in drainage area also leads to sharp contrasts in water and sediment flux between trunk and tributary channels. The occurrence of hanging valleys in actively uplifting fluvial landscapes such as the Central Range of Taiwan suggests that the most common parameterizations of bedrock erosion, which typically assume a monotonic positive correlation between channel gradient and incision rate, may be violated in very steep channels. In addition, hanging valleys could greatly increase the response time of landscapes to tectonic perturbations since catchments above these tributary mouths will be insulated from these perturbations until a new suite of processes (e.g., weathering and rock mass failure) wear through the hanging valley lip. The results of this study underscore the need for a more complete understanding of bedrock erosion processes and the incorporation of process transitions and threshold conditions into landscape evolution models.