Fluvial geomorphology and paleofloods in arid central Australia

Abstract

This thesis examines the fluvial geomorphology of the Todd River(9,300 km2) in arid central Australia. It describes the morphology, motphostratigraphy, dimentary characteristics and chronology of the modern flood plain and paleoflood complexes, and discusses the geomorphic effects of high magnitude floods and the subsequent recovery and adjustment of the river. The Todd River has two kinds of flood plains: confined and relatively unconfined. Both have a complex morphostratigraphy which reflects the variable discharge regime of this semi-arid to arid,summer rainfall region. The flood plains are episodicallv eroded and reconstructed during flood events and both processes may occur in different parts of the system during a single event. The principal mechanisms of flood plain destruction are channel widening, flood plain stripping, swirl pit,flood and back channel scour. The principal mechanisms of flood plain construction are the deposition of insets, flood plain veneer, channel and whirl pit fill, channel abandonment and the formation of simple or compound bars which can become islands. Channels in the relatively unconfined reaches tend to shift position by avulsion; channels in confined reaches change width and migrate laterally. The flood plains have stepped surface morphologies with prominent flood channels on their surfaces and back channels often occur between surface elements at different elevations. Swirl pits develop around large River Red Gums (Eucalypcus camadulensis). The principal morphostratigraphic units include surface and back channel fills, channel and flood plain insets, surface and buried pa1eoflood and flood plain remnants, veneer and swirl pit fill. Morphostratigraphic models of confined and relatively unconfined flood plains are presented. The mud, sand and gravel Flood plain sediments frequently ate not deposited in simple fining upward sequences, as coarse sediments are transported across flood plains during floods. Buried erosion surfaces are common within flood plain deposits. Rippled bar-top and dipping bar face sand and gravel, flood couplets, sand sheets mud drapes, and mud balls are typical sedimentary structures. Dating by radiocarbon and optically stimulated luminescence shows that sediment is stored in the active flood plain for only a few hundred years before being transported and redeposited. Reaches of temporary sediment storage sometimes are subject to extreme cha11nel widening, often located upstream of bedrock constrictions or clown-stream of tributaries, or where the channel boundary is composed of erodable paleoflood sediments. Paleofloods greater than observed historical floods flowed from gorges through the MacDonnell Ranges as sheet floods depositing transverse bar fields, at H.l channels flowed across fans forming braided 12 km splays that include large longitudinal bars. Flows were diverted around bedrock ridges forming erosional moats, macroturbulent scour trains and 4-8 km wide, braided expansion bars with marginal channels,downstream from constrictions, high longitudinal dunes were eroded and the most northerly region of the Simpson Desert dune field remains as streamlined remnants. Slack water sediments were deposited in caves,tributary mouths and back water sediments were deposited in the longitudinal dune swales. Paleoflood deposits are widespread owing to repeated channel avulsion and inundate a larger area than if channels had remained fixed. The terminal floodout of the catchment has been repositioned at least twice. Dating indicates that the highest magnitude flows occurred between 14,000 and 4,000 Ill' decreasing in magnitude since the mid Holocene. The dating of paleoflood sediments by optically stimulated luminescence and radiocarbon has extended the paleoflood record of central Australia back to 27 but there is no indication of large scale flooding during the last major arid phase 24,000 to 150 000 BP,and floods have occurred randomly Since -14,000 BP. Some paleofloods may be coeval with events in northern Australia but dating is not sufficiently precise to prove this. The adjustment and recovery of the Todd River to the most recent paleofloods is ongoing today and the recovery time appears to exceed the paleoflood recurrence interval.