Evidence for flocculation in glacier-fed Lillooet Lake, British Columbia.

This study provides evidence of in situ flocculation in a glacier-fed freshwater lake. Direct observation of flocculated particles using microscopy provides the strongest evidence to establish that clastic suspended sediment in Lillooet Lake is flocculated. Flocs in Lillooet Lake exhibit a hierarchical size structure, with primary particles (<4 microm) joining to create microflocs (10-35 microm), and both primary particles and microflocs joining to create macroflocs (200-280 microm). In situ laser particle sizing reveals primary particles and microflocs dominate in the epilimnion, a zone susceptible to shear induced by wind or inflow and exhibiting the highest suspended sediment loads. Macroflocs dominate in the hypolimnion, a more quiescent zone and exhibiting the lowest suspended sediment loads. Flocculated particles are invisible to traditional methods of sediment analysis in glaciolacustrine settings. Flocculation is a plausible explanation for accelerated sedimentation of the clay-sized sediments known to dominate the size distribution of varves in other glacier-fed lakes.