Alluvium of antiquity: Polycyclic terraces in a confined bedrock valley

Confined river valleys are not the localities where long term preservation of alluvium would be expected. The 25 km long low gradient (0.0014 m.m− 1) confined valley setting of the Shoalhaven River has archived alluvium of middle Pleistocene age to maintain a relatively uniform channel as an efficient conduit for a wide range of flows in a confined bedrock valley of variable morphology. Single-grain optically stimulated luminescence (OSL) dating has identified polycyclic terraces up to 193 ka in age (marine isotope stage [MIS] 7) with lower terrace remnants dating from 173–140 ka (MIS 6) and 106 ka (MIS 5). Holocene alluvium 2–3.5 ka in age caps these old Pleistocene units and a well-constrained combination of one-dimensional and two-dimensional hydraulic modelling demonstrates that these polycyclic terraces are clearly within reach of the modern hydrological regime. The 106 ka terrace at 17 m above low flow is inundated by floods recurring on average every ~ 20 years, and the 140–193 ka terraces at 20–22 m are overtopped every 50–100 years. These ancient diachronous landforms exhibit complex depositional histories and are on-lapped by longitudinal benches of modern sand and gravel. Their polycyclic nature appears to be a response to flow reduction, using alluvium to adjust the boundary of the otherwise inflexible morphology of a bedrock gorge.