Sedimentary records of late Holocene floods along the Fitzroy and Margaret Rivers, Western Australia

Slackwater sediments and palaeostage indicators record floods occurring prior to discharge gauging along the Fitzroy and Margaret Rivers. In upper Geikie Gorge along the Fitzroy River, preserved sequences record six floods ranging from 5000 to 30 000 m³s^‐1 during the last 2000 years. Approximately 13 floods between 2000 and 20 000 m³s^‐1 are recorded in an unnamed gorge along the upper Margaret River during the last 4000 years. These floods are at the upper limit of world‐wide discharge‐drainage area curves.

The length and resolution of the flood record preserved in slackwater deposits depend on the exposure of flood sediments to post‐depositional weathering, the nature of flood sedimentation, channel geometry and stability, and the frequency of floods. Conditions along the Fitzroy and Margaret Rivers are well‐suited to preserving long, fairly accessible sedimentary flood records, in comparison to sedimentary records described from similar bedrock channels in the tropical 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 systematic gauge records from these basins may not adequately represent extreme discharges.

Key words:

• Fitzroy River
• Margaret River
• palaeofloods
• slackwater deposits