Extreme sediment delivery events recorded in the contemporary sediment record of a montane lake, southern Coast Mountains, British Columbia

Abstract

The extreme sediment delivery regime of a montane catchment was examined through the analysis of recent lacustrine varve deposits recovered from a high-density sampling program. Years of anomalously high sediment yield from the Green Lake watershed were identified over a 70-year period (1930–2000) based on whole-lake varve deposit volumes. Intra-annual sediment delivery events were categorized and described for those anomalous years using historical hydroclimatic data and the stratigraphic record observed within inflow proximal sediment cores. Extreme sediment delivery coincided with high discharge conditions and elevated sediment availability resulting from (1) rapid glacier recession of the early twentieth century, (2) late-summer and autumn rainstorm-generated floods, and (3) freshet floods caused by unusual snowmelt conditions. The thickness and physical characteristics of varves vary among years that experienced different types of moderate and extreme sediment delivery events in relation to the defined average-regime depositional model. Several hydroclimatic factors can interact to produce composite varve units of anomalous thickness. In some cases, geomorphic events, such as landslides and channel changes, contributed to extreme sediment delivery in the Green Lake catchment. The interaction of these geomorphic effects, coupled with the variable nature of associated hydroclimatic functions, complicate paleoenvironmental assessments based on the lacustrine varve record.