Formation of fluvial hanging valleys: Theory and simulation

Although only recently recognized, hanging tributary valleys in unglaciated, tectonically active landscapes are surprisingly common. Stream power-based river incision models do not provide a viable mechanism for the formation of fluvial hanging valleys. Thus these disequilibrium landforms present an opportunity to advance our understanding of river incision processes. In this work, we demonstrate that thresholds apparent in sediment flux-dependent bedrock incision rules provide mechanisms for the formation of hanging valleys in response to transient pulses of river incision. We simplify recently published river incision models in order to derive analytical solutions for the conditions required for hanging valley formation and use these results to guide numerical landscape evolution simulations. Analytical and numerical results demonstrate that during the response to base level fall, sediment flux-dependent incision rules may create either temporary or permanent hanging valleys. These hanging valleys form as a consequence of (1) rapid main stem incision oversteepening tributary junctions beyond some threshold slope or (2) low tributary sediment flux response during the pulse of main stem incision, thus limiting the tributary's capacity to keep pace with main stem incision. The distribution of permanent and temporary hanging valleys results from four competing factors: the magnitude of base level fall, the upstream attenuation of the incision signal, the lag time of the sediment flux response, and the nonsystematic variation in tributary drainage areas within the stream network. The development of hanging valleys in landscapes governed by sediment flux-dependent incision rules limits the transmission of base level fall signals through the channel network, ultimately increasing basin response time.