Imaging active mass-wasting and sediment flows on a Fjord Delta, Squamish, British Columbia

Abstract

An active fjord head delta in Squamish British Columbia, was selected as the location for a repetitive multibeam survey program to monitor temporal evolution of the prodelta morphology. Daily resurveys in 2011 established the style and extent of submarine mass movements, their typical periodicity and the conditions associated with the most active periods. This has now been followed by an hourly resurvey program in 2012 during those most active periods to actually witness the progression of activity immediately preceding, during and subsequent to a singular event. The delta front in depths of 20-50 m is often the apparent start point for trains of sequential erosive and depositional events associated with upslope bedform migration along prodelta channels. Heavy targets on the channel floors were monitored in 2011 and indicated rare, abrupt down channel displacements of a few hundred metres, indicating that a small subset of events involved bulk translation of the seabed. In 2011, a bottom-mounted ADCP beyond one channel mouth recorded clear turbidity current events for a subset of the channel bedform migration periods. In 2012, using multibeam water column imaging and a rapidly dipping towed optical backscatter probe, the evolution of a descending suspended sediment plume below the overlying river plume was monitored on an hourly basis. Towards low water, that descending plume was seen to occasionally feed a near seabed higher suspended sediment layer. On the development of this layer, the water column imaging revealed a thin basal flow that lasted about an hour and corresponded directly with the period of migration of the channel floor bedforms. Delta-lip failures are associated with the upslope end of about half of the bedform trains suggesting an alternate initiating mechanism.