Pool-fills: a window to palaeoflood history and response in bedrock-confined rivers

Channel‐scale sedimentary units associated with bedrock‐controlled riffle‐pool morphology are examined in detail along Sandy Creek gorge, an ephemeral stream in arid south‐eastern central Australia. Pool‐fills comprise cut‐and‐fill assemblages of poorly sorted sediments ranging in texture from muds to boulders. Five unit types are defined based on particle size, sedimentary structures, geometry and bounding surface character: (1) coarse‐grained bar platform; (2) fine‐grained bar supraplatform; (3) fine‐grained pool‐fill; (4) fine‐grained bench; and (5) modern pool‐fill. The last coarse‐grained unit currently lining the pools suggests an altered sedimentation style over the post‐settlement period (post‐ad 1860s). Situated at bedrock valley constrictions, pool‐fills are compared with other sedimentary units associated with recirculating currents: eddy bars and slackwater deposits. But only the fine‐grained bench units reflect eddy recirculation; the pool‐fills are principally forced‐bars associated with bedrock‐controlled or ‘forced’ riffle‐pool morphology. A late Holocene palaeoflood history is proposed based on radiocarbon ages from the pool‐fills: multiple phases of cut‐and‐fill activity were preceded by a superflood 3400–1900 years ago that eroded the pool‐fills to bedrock. The resilience of the pool‐fills was illustrated by the passage of a 1‐in‐100‐year flood in 1992, which caused only minor erosion. The presence of pool‐fills may provide a window to past phases of river activity that cannot be extracted from either historical records/observations or palaeoflood slackwater sediment analyses. The formation and sedimentary preservation potential of these landforms reflect a combination of hydraulic and structural influences, but the occurrence of high‐magnitude floods exerts the dominant control.