SEDIMENTARY RECORDS OF LATE HOLOCENE FLOODS ALONG THE FITZROY AND MARGARET RIVERS, WESTERN-AUSTRALIA

Slackwater sediments and palaeostage indicators record floods occurrin g prior to discharge gauging along the Fitzroy and Margaret Rivers. In upper Geikie Gorge along the Fitzroy River, preserved sequences recor d six floods ranging from 5000 to 30 000 m3s-1 during the last 2000 ye ars. Approximately 13 floods between 2000 and 20000 m3s-1 are recorded in an unnamed gorge along the upper Margaret River during the last 40 00 years. These floods are at the upper limit of world-wide discharge- drainage area curves. The length and resolution of the flood record pr eserved in slackwater deposits depend on the exposure of flood sedimen ts to post-depositional weathering, the nature of flood sedimentation, channel geometry and stability, and the frequency of floods. Conditio ns along the Fitzroy and Margaret Rivers are well-suited to preserving long, fairly accessible sedimentary flood records, in comparison to s edimentary records described from similar bedrock channels in the trop ical regions of Queensland and the Northern Territory. The Fitzroy and Margaret River flood deposits indicate that, similar to other rivers with highly variable hydrologic regimes, the relatively short-term sys tematic gauge records from these basins may not adequately represent e xtreme discharges.