DOWNSTREAM GEOMORPHOLOGIC CHANNEL CHANGES ON DISTRIBUTIVE FLUVIAL SYSTEMS AND IMPLICATIONS FOR INTERPRETING THE ROCK RECORD

Analysis of over 400 fluvial megafans in aggradational continental sedimentary basins reveals that downstream channel and floodplain changes on these distributive fluvial systems (DFS) tend to behave in predictable ways to climate in the catchment and receiving basins. The changes in channel dimensions can be explained in geomorphologic and hydrologic terms of the river’s response to variation in discharge and valley slope in order to maximise the potential energy to transport the water and sediment supply. Contrary to the many examples discussed in hydrogeomorphic literature for tributary fluvial systems, observations suggest that intrinsic thresholds lead to the breakdown of the main trunk channel into smaller distributary channels and eventual disintegration of the majority of channelized flow at the DFS termination. These thresholds are related to downstream decreases in discharge and stream power, and downstream fining of sediment load. Lateral accretion deposits which are common components of the upstream sinuous channel and floodplain decrease in importance to a threshold point, beyond which vertical accretion deposits dominate the lower distal reaches of the system. In-channel and bank-top sediment accumulation increase as stream power declines, contributing to overbank and distributary flow losses and further loss of stream power leading to more avulsive behaviour in the system. Essentially, the underlying physical-processes, or hydraulic principles, of the channels on these distributive fluvial systems are identical to those for channels in tributary fluvial systems. However, current facies models based on our understanding of tributary fluvial systems are inadequate to interpret the apparently counter-intuitive downstream behaviour and signature of DFS as preserved in the rock record. New generic facies models are presented derived from observations of remotely sensed imagery for the distribution of sedimentary facies on modern DFS in a range of environmental settings.