Floodplain stripping: another form of adjustment to secular hydrologic regime change in Southeast Australia

Abstract

In Southeast Australia it has been found that river regimes over the last 200 years (the period of European settlement) have alternated between flood and drought domination. Flood-dominated regimes (FDRs) are periods of up to 50years of frequent, high-magnitude floods, whereas drought-dominated regimes (DDRs) are characterised by floods of much lower frequency and lower magnitude. Responses to these secular changes in regime have so far been mainly studied as channel changes. However, it is now apparent that, in high-energy reaches where resistant channel perimeters are slow to adjust to such changes, overbank adjustment in the form of alluvial stripping can occur, especially following the removal of dense floodplain forests by Europeans. Three types of stripping are described: across meander chutes, parallel chutes and convex bank erosion. Examples of these are presented for three coastal valleys. Evidence reveals that they are recent forms and originate after the late 1940s shift to an FDR. Growth has continued in this regime. In the Clarence River, there is a suggestion that both high- and low-level stripping at some sites may have been in the late nineteenth century FIR. At Charity Creek on the Manning River, there is also evidence of a large parallel chute, active with two ponds, in 1890. This healed in the 1901–1948 DDR and has been re-stripped in the present regime. Such findings conflict with interpretations by Nanson (1986, Geol. Soc. Am. Bull., 97: 1467–1475) of such forms as evidence of cyclic floodplain evolution in laterally constrained valley-floor troughs.