CHANNEL BIFURCATION AND SLOPE ON BRAIDED DISTRIBUTIVE FLUVIAL SYSTEMS

Large (> 30 km) distributive fluvial systems (DFS), also known in the literature as megafans and fluvial fans, are the result of sediment deposition that occurs when rivers exit the confinement of mountain valleys and become laterally mobile in broad sedimentary basins. DFS dominate deposition in continental sedimentary basins, thus understanding mechanisms that control DFS development will be key to understanding basin-scale fluvial architecture. Braided DFS are characterized by a radial pattern of channels from an apex of channelbelts that evolved through bifurcation and avulsion. For this study, bifurcation is the splitting of channelbelts into two or more smaller, active channelbelts that are separated by floodplain deposits. If discharge in one channel belt falls below a critical value, it is abandoned and the bifurcation may develop into an avulsion. This study focuses on two aspects related to understanding river channel network patterns and evolution in sedimentary basins: 1) whether a correlation exists between long-profile and cross-profile slopes and mechanisms that control channel bifurcation on large braided DFS and 2) construction of a database that can be efficiently queried to carry out research on fluvial systems in sedimentary basins. A preliminary analysis of 32 large DFS with braided bifurcating planforms was carried out using Shuttle Radar Topography Mission (SRTM) digital elevation models (DEMs) and LANDSAT satellite imagery. 16 of the 32 braided DFS bifurcate at the apex, of which 12 (75%) have a slope of < 1˚ around the bifurcation point. Of the remaining 16 braided DFS that bifurcate below the apex, all 16 (100%) have a slope < 1˚ around their bifurcation points. For the 32 bifurcating braided DFS sampled, channel bifurcation occurred on slopes < 1˚ 88% of the time. Although the mechanisms that cause bifurcation on braided DFS is presently unknown, understanding these mechanisms is important because floodplain material is often observed between the active channelbelts. Continued development of the database on the remaining 269 braided large DFS identified around the world will help test whether this slope-bifurcation relationship is consistent on most DFS.