Dynamics of River-Sediment Delivery and Turbidity Current Flow Observed in Lillooet Lake, British Columbia

Abstract

A study linking sediment-discharge dynamics and lake-bottom properties to plunging river flow was undertaken at Lillooet Lake, British Columbia. This study was initiated to understand the dynamics of turbidity currents that plunge off the front of the Lillooet-Green River delta and how those factors have an influence on turbidity current development. Understanding how turbidity currents behave is important to understanding the sedimentary budget and hydrology of lakes and reservoirs. To achieve the objectives of understanding how sediment-discharge dynamics and lake-bottom properties influence the development of turbidity currents in Lillooet Lake, field monitoring was undertaken for 10 days in July 2015. Field monitoring consisted of monitoring river stage (discharge), river temperature, and suspended sediment concentration as well as monitoring lake currents using an aDcp and a moored lake-temperature array. Water profiles were taken along the front of the delta. Changes to the subaqueous delta were monitored using single-beam sonar and the surface of the delta was monitored using an Unmanned Aerial Vehicle. During this study, record-low discharge and lake levels were encountered which had a detrimental effect on monitoring flow properties. The resultant analysis provided several key observations based on the data collected during this study, given these conditions. (1) The fluvial sediment-discharge relationship had negative hysteresis, possibly due to the decoupled nature of the sources of sediment and meltwater in Lillooet-Green River. (2) The lower boundary of turbidity current showed pulsating velocity through time, thought to be the result of shearing at the boundary between the current and ambient lake water. (3) A benthic trough was observed in front of the plunge line lobe that is believed to be a path for preferential flow as well as an eroded surface from turbidity currents originating from Lillooet-Green River delta. The findings indicate that even at low-flows the sediment laden water plunging off the Lillooet-Green River delta can produce interflows and underflows described as turbidity currents. Plunging water appears to be more dynamic than has previously been shown which is illustrated by the gradient of water velocity and changes to the delta slopes of Lillooet-Green River delta.