|
Below the channel floor or thalweg (the deepest part of the channel), coarsest material is sorted out and left behind on the stream bed. Lying just above the basal erosional surface (or scour base), the channel lag consists of mudclasts and blocks from bank erosion, plant debris, boulders, and bed-load sand and gravel. In the lower reaches of the river where coarse material may not be available, the deposits usually contain water-saturated logs and twigs, mud pebbles, large blocks of unconsolidated sediment, and dead organisms and plants. Scour pockets (a depression produced in the stream bed by channel scour) provide good environments for lag deposits.
The channel lag deposits are often overlain by either channel floor or lower point bar sands and gravels. These deposits are usually discontinuous lenticular patches and vary in thickness from one layer of coarse material to several meters.
|
