Land-To-Sea Sediment Fluxes From a Major Glacial Lake Outburst Flood Were Stepped Rather Than Instantaneous

Glacial lake outburst floods can transport large volumes of sediment. Where these floods reach the coastline, much of the particulate matter is delivered directly to the marine environment. It has been suggested that offshore deposits, specifically in fjord settings, may provide a faithful record of past outburst flood events. However, a lack of observations means that the mechanics and the timing of sediment transport offshore following a glacial lake outburst event remain poorly constrained. Here, we document the changes in sea surface sediment dynamics following the 28 November 2020 Elliot Lake outburst flood in British Columbia, which transported ∼4.3 × 10⁶ m³ of sediment into an adjacent fjord (Bute Inlet) as a deep nepheloid layer directly following the event. However, analysis of sea surface turbidity using in situ measurements and satellite-derived estimates reveals that changes in fjord-head surface turbidity in the months following the major flood were surprisingly small. The highest measured sea surface turbidity instead occurred 5 months after the initial outburst flood. This delayed increase in seaward sediment flux coincided with the onset of the spring freshet, when the discharge of the rivers feeding Bute Inlet increases each year. We suggest that large quantities of sediment were temporarily stored within the river catchment and were only remobilized when river discharge exceeded a threshold level following seasonal snowmelt. Our results reveal a temporal disconnect, where onshore to offshore transfer of sediment is stepped following a glacial lake outburst flood, which could complicate the sedimentology of subsequent deposits.