Geomorphic elements on modern distributive fluvial systems

Analysis of over 400 fluvial megafans (> 30 km in length) in aggradational continental sedimentary basins reveals that geomorphic channel and floodplain changes on these distributive fluvial systems (DFS) generally behave in predictable ways with increasing distance from the apex. These changes can include: a decrease in discharge, a decrease in bed material transport and calibre of sediment, a decrease in stream power, an overall decrease in channel width, an overall decrease in channel depth, an increase in avulsive behaviour, and sinuosity becomes more variable.Three generic geomorphic element models are proposed – reflecting observed changes in channel behaviour – based on measurable changes in channel width and planform characteristics with increasing distance downstream. The three models are derived from (1) a single braided channel that bifurcates downstream into low sinuosity channels; (2) a dominant, sinuous, single-thread channel that anabranches and bifurcates with distance downstream, creating smaller channels with varying sinuosity; and (3) a dominant multi-thread channel that anabranches and bifurcates with distance downstream, creating smaller channels with varying sinuosity.The changes in fluvial behaviour and landforms on DFS are in response to variable discharge and sediment supply ratios from the upstream catchment. In contrast to examples described in hydrogeomorphological literature for tributary fluvial systems where channel dimensions tend to increase downstream, observations from DFS suggest that – where the formative DFS channel does not retain the same dimensions – intrinsic geomorphic thresholds lead to the breakdown of the main trunk channel into smaller anabranching and distributary channels with distance downstream; in some instances the majority of channelised flow at the DFS termination may even be disintegrated. The observed range of termination types and floodplain soils for each DFS type are interchangeable dependent on local conditions. The modern geomorphic elements and floodplain soils are dependent on climate in the upstream catchment and in the downstream receiving sedimentary basin.