Likely Very Little Sediment (better word choice)I suspect there is very little sediment. The waste rock is essentually sitting on a rocky lake bottom.
This is an enclosed alpine water body at high elevation with no inflow river bringing in sediment and surrounding sparse conifer cover and barren rock. These types of water bodies have very little sediment.
Check out the diagram and picture of the lake posted on Skeena's website.
This lake would also be easy to drain by simple siphoning