Flood stratigraphies in lake sediments: A review

Records of the frequency and magnitude of floods are needed on centennial or millennial timescales to place increases in their occurrence and intensity into a longer-term context than is available from gauged river-flow and historical records. Recent research has highlighted the potential for lake sediment sequences to act as a relatively untapped archive of high-magnitude floods over these longer timescales. Abyssal lake sediments can record past floods in the form of coarser-grained laminations that reflect the capacity for river flows with greater hydrodynamic energy to transport larger particles into the lake. This paper presents a framework for investigating flood stratigraphies in lakes by reviewing the conditioning mechanisms in the lake and catchment, outlining the key analytical techniques used to recover flood records and highlighting the importance of appropriate field site and methodology selection. The processes of sediment movement from watershed to lake bed are complex, meaning relationships between measureable sedimentary characteristics and associated river discharge are not always clear. Stratigraphical palaeoflood records are all affected to some degree by catchment conditioning, fluvial connectivity, sequencing of high flows, delta dynamics as well as within-lake processes including river plume dispersal, sediment focussing, re-suspension and trapping efficiency. With regard to analytical techniques, the potential for direct (e.g., laser granulometry) and indirect (e.g., geochemical elemental ratios) measurements of particle size to reflect variations in river discharge is confirmed. We recommend care when interpreting fine-resolution geochemical data acquired via micro-scale X-ray fluorescence (μXRF) core scanning due to variable down-core water and organic matter content altering X-ray attenuation. We also recommend accounting for changes in sediment supply through time as new or differing sources of sediment release may affect the hydrodynamic relationship between particle size and/or geochemistry with stream power. Where these processes are considered and suitable dating control is obtained, discrete historical floods can be identified and characterised using palaeolimnological evidence. We outline a protocol for selecting suitable lakes and coring sites that integrates environmental setting, sediment transfer processes and depositional mechanisms to act as a rapid reference for future research into lacustrine palaeoflood records. We also present an interpretational protocol illustrating the analytical techniques available to palaeoflood researchers. To demonstrate their utility, we review five case studies of palaeoflood reconstructions from lakes in geographically varied regions; these show how lakes of different sizes and geomorphological contexts can produce comprehensive palaeoflood records. These were achieved by consistently applying site-validated direct and proxy grain-size measurements; well-established chronologies; validation of the proxy-process interpretation; and calibration of the palaeoflood record against instrumental or historical records.