The geomorphology and sedimentology of terminal fluvial systems

The geomorphology and sedimentology of terminal fluvial systems. Fluvial systems operating within drylands commonly experience downstream discharge reduction due to infiltration, evaporation and limited tributary inputs. Sediment bodies developed within the distal zones of rivers that do not drain into the sea or a lake (terminal fluvial systems) are currently represented by the terminal fan facies model. This model summarises the development of a distally thinning and fining sedimentary wedge from a coeval network of low sinuosity distributary channels as induced by the sub aerial termination ofchannelised flow under a dryland climate regime.. Extensive review of sediment fan bodies located within modern drylands highlights pronounced disequilibrium between planform character and present ephemeral flow conditions. Out of eighty documented fluvial systems no convincing examples fit the terminal fan model, including two commonly cited analogues used to support this model. In order to fully evaluate the terminal fan concept and redress the current imbalance in modern analogue studies, field work was undertaken to characterise sub aerial fluvial system termination within a single physiographic province - the Basin and Range rift complex of the southwestern U.S.A. Documentation of the potential range in fluvial style and character within this modern dryland environment is provided by the detailed study of seven terminal fluvial systems. Basin and Range terminal fluvial systems demonstrate strong geomorphic form inheritance. Fan landforms observed within medial and distal reaches of these systems predominantly represent relic Late Pleistocene highstand delta bodies into which the modern system is inset. Active terminal reaches operate within basin centre playa environments where shallo~ gradients induce frequent avulsion and the generation of composite lowstand fan bodies located downstream of lateral system confinement. These terminal features record non-coeval channel activity and the dominance of sinuous channel forms. Morphometric trends distinguish a general downstream reduction in channel scale characterised by the development of progressively narrower and shallower channel forms. Channelised flow is maintained within proximal and medial reaches but does not dominate distal reaches where sheetflow discharge is readily attained. Concomitant reductions in channel capacity and competence control the volume and calibre of fluvial material supplied to basinal environments. Progressive downstream thinning is associated with selective deposition and general basinwards sediment fining and sorting. Terminal reaches transport negligible bedload material and display a comparable depositional record to that generated by background playa sedimentation. Identified similarities with the terminal fan model include downstream loss of channel definition, sediment thinning, fining and improved sorting. Conversely, coeval distributive flow is not observed, constituent channels record moderate to high sinuosity and negligible fluvial material reaches basin centre locations. Basin and Range systems are principally responding to streampower reduction controlled primarily by gradient and enhanced by discharge attenuation. Sub aerial termination dominates due to the absence of basin centre lacustrine bodies; a condition forced by limited discharge supply from catchment reaches, compounded by transmission losses and maintained by excessive evaporation from extensive, low elevation flat playa surfaces. In conclusion, fluvial fan landforms generated exclusively from discharge attenuation do not characterise modern dryland environments. Selection of modern analogue systems for use in the interpretation and prediction of ancient fluvial successions must acknowledge the influence of high frequency and high magnitude climate fluctuations upon modern fluvial geomorphology. Key to this is an appreciation of modern processform disequilibrium and the identification of inherited planform characteristics.EThOS - Electronic Theses Online ServiceGBUnited Kingdo