Patterns and processes of channel and floodout adjustment in a discontinuous dryland river, semi-arid eastern Australia

Abstract

Alluvial rivers in semi-arid and arid landscapes are prone to episodic geomorphic adjustments related to long-term sediment accumulation and occasional reworking by flash floods. However, few studies document the rates, patterns and processes leading to channel breakdown, particularly avulsion and floodout. These intrinsically driven adjustments may pose a risk to water resource availability through redistribution of water and sediment on the floodplain. To address this knowledge gap, channel change associated with avulsion and flooding in the last ∼30 years were quantified for Little Faulkenhagen Creek, a dryland river of the Murray-Darling Basin, Australia. The semi-arid, low gradient catchment promotes transmission losses, which with loss of valley confinement leads to channel adjustment and contraction to occur where water, sediment and energy are spread across the floodplain. Pronounced downstream changes occur in channel capacity (i.e., reductions in bankfull width, depth and area) and hydrology (i.e., reductions in bankfull discharge and stream power) as the river enters the floodout zone. Vegetation roughness increases in and around the floodout, where shrubs and river redgum trees line and encroach upon the channel. Mapping and analysis of hydro-geomorphological parameters using cross sections from 1991 to 2021 reveal recent changes in channel capacity, discharge and stream power. The upper reach has experienced substantial channel widening from erosion, while the lower reach leading into the floodout has contracted because of sediment deposition within the channel. Unchannelised floodwaters are deflected around a raised alluvial ridge and over the floodout eventually dropping into several migrating upstream knickpoints (head-cutting gullies) with retreat rates up to 32 m a−1. Therefore, both the existing channel slope (i.e., decline) and potential avulsion courses (i.e., increase) are affected by reduced energy and enhanced sedimentation. The net result of upstream in-channel sedimentation and aggradation on the floodout and downstream incision and gullying is that the floodout has migrated ∼1 km upstream and the river is at risk of a secondary avulsion as gullies retreat towards the main channel, potentially circumventing the floodout within ∼11 years. An erosion cell conceptual model is suitable for this system, where sediment from the local upstream production (erosion) zone is transferred downstream to a sink (deposition) zone. Erosion and deposition will fluctuate over the long-term, leading to periods with dominantly incisional or aggradational processes. Cycles of erosion and deposition will vary over time and external forces may influence a wholesale shift to a new fluvial state. Understanding the context and patterns of recent dryland river adjustments, as well as the dominant processes and their variability, can assist water and land management.