The Hydrology, Geomorphology and Quaternary Palaeochannels of the Lachlan Valley, New South Wales

Abstract

This thesis examines the present-day and Late Quaternary fluvial geomorphology and hydrology of the upper Lachlan Valley in central western New South Wales. The sediments and morphology of the channel and floodplain are described from its principal alluvial reaches: the confined, single channel reach from Cowra to Gooloogong, the unconfined, single-channel reach between Gooloogong and Cadow, and the anabranching reach on the Condobolin Plain. Present-day channel and floodplain morphology in single-channel reaches of the Lachlan River is dominated by the highly variable hydrologic regime, which is accentuated in confined reaches of the river, but is also influenced by inherited channel forms. In confined reaches, where the effective floodplain is <lkm wide, the floodplain is a high-energy surface dominated by large flood features, which elsewhere in the world are attributed to catastrophic flood flows, but which here are forming under the normal regime of the river. Floodplains are characterised by elliptical scour scars, zones of floodplain stripping, chutes and chute bars, compound flood levees and parallel flood channels. In contrast, the channel itself contains low-energy features such as fine-grained, accreting bench deposits which point to remarkable stability of the channel position. Floodplain deposits resulting from fluvial processes operating at a variety of scales are poorly sorted and become coarser with height above the river bed. The hydrologic character of the river, described from streamflow and historical flood records, is consistent with the sedimentary and morphological evidence of a flood-dominated floodplain, and dating evidence indicates that this has been a long-standing characteristic of this river system in this location. In less confined reaches, large flood features are more subdued and channel inheritance locally controls meander patterns and channel slope. Inherited influences increase downstream as confinement of flood flows is less, and channels may become entrenched within highly sinuous palaeomeanders, developing only a narrow floodplain. The effect of variable streamflow is still exhibited in chute channels, scour complexes and zones of stripped floodplain, although the channel itself is governed by bankfull flows. Lateral channel activity increases in the downstream direction, reflected in both the sinousity and width of the active floodplain. These channel and floodplain characteristics continue throughout the anabranching reach on the plains. Hydraulic geometry relationships in both single and anabranching reaches were constructed from streamflow records and surveyed channel cross-sections, and include sediment parameters. The relationship between channel shape and silt-clay content on the Lachlan differs from that described elsewhere. The exponents found for depth and velocity as a function of discharge also exhibit notable departures from trends reported for rivers elsewhere in the world, as the standard relationships are based on rivers which increase in size downstream, while both slope and particle size decrease. The Lachlan presents a reverse situation, where discharge, slope and sediment size all decrease downstream, and illustrates the need to develop regional rather than global relationships to estimate bankfull discharges for ungauged channels with a given set of characteristics. Palaeochannels were described for the Lachlan Valley downstream from Cowra, where they are associated with two major alluvial terraces, and on the Plains downstream from Forbes, where they intersect with each other and with the modern floodplain. Three discrete fluvial systems were defined. The Gulgo Fluvial System is characterised by relatively narrow and deep channels and has a stable, anabranching pattern. A single TL date from point bar sediments of this system suggests these channels were active by at least 57,000 yrs ago and were replaced by channels of the Ulgutherie Fluvial System, characterised by sinuous, regular and scrolled meanders with wide and shallow channels. OSL dates on channel sediments and an overlying source-bordering dune suggests Ulgutherie channel construction and aeolian deflation of fluvial sediments were underway by 34,000 yrs and the system may have declined soon afterwards. Discharge estimates based on several formulae, including relationships developed for the modern river, indicate bankfull discharges of the Ulgutherie system were at least 4 to 7 times those of the present-day river. Ulgutherie channels were replaced by channels of the Nanima Fluvial System, which had a similar morphology to the Ulgutherie channels, with scrolled, regularly sinuous meanders, but smaller channel capacities and width-depth ratios, and sediment characteristics more similar to the modern Lachlan. Radiocarbon dates indicate Nanima channels were active by 6,000 yrs, if not before, and were replaced by smaller, irregularly meandering channels of the present fluvial system soon after 3000 yrs ago. Discharge reconstructions of Nanima channels indicate they carried bankfull discharges 1.5 to 2 times that of the present river. The higher discharges associated with meandering palaeochannels in the Lachlan Valley around 34,000 yrs and from >6,000-3,000 yrs is in good agreement with lake-level and other environmental records within the catchment. Show more