Flood channel morphology of a quiet river, the Lachlan downstream from Cowra, southeastern Australia

Abstract

Detailed geomorphic mapping and sediment analyses of floodplain features were undertaken on a 30-km reach of the Lachlan River in southeastern Australia. The reach is located in the transitional zone between the southern Great Dividing Range and extensive alluvial plains to the west. The channel resembles other fine-grained, low-energy, alluvial channels; it is muddy, narrow and deep, and meanders irregularly. It contains fine-grained accreting bench deposits, and, in places, is influenced by inherited fluvial forms, both of which indicate long-term channel stability. By contrast, the floodplain exhibits features that have been attributed elsewhere to catastrophic flood flows, including elliptical scour scars ("swirl pits"), cobble and gravel deposits, large chutes and chute bars, zones of floodplain stripping, flood levees and multiple flood channels. Sorting and texture of floodplain sediments generally increases with height above the bed, although deposits are variable and poorly sorted. Indications of recent fluvial activity at several levels above the bed are confirmed by aerial photographs dating from 1954, although changes to the channel position have been small. The alluvial facies model displays a compound floodplain, in which a stable, low-energy channel is inset within a high-energy, flood-dominated floodplain. This interpretation is consistent with streamflow and documentary evidence of recurrent large floods since 1839, which have a return period of <10 years on both the partial and annual series. Natural streamflow is characterised by high annual variability and a steep flood growth curve. In such systems, flow variability, together with stream power and sediment characteristics, appears to be an important determinant of floodplain type.