Wednesday, April 18, 2018

Self Armored River Bluff

We recently traversed along the base of a late-Pleistocene terrace consisting of glacial till and ice proximal meltwater. The terrace formed by incision and erosion by the Nooksack River to create a river bank bluff. The bluff is about 25 feet high. There are numerous large cobbles and boulders at the base of the bluff along the river edge derived from cobbles and boulders that have fallen from the terrace slope forming a lag of very course sediment along the edge of the river. Many of the boulders are much larger that the cobble sediment within the gravel and cobble bars within the river area. 

Cobbles and boulders in the river bluff slope 

The large cobbles and boulders accumulated at the base of the slope along the river appeared to be limiting the erosion along the terrace. The bare slope above shows that some erosion has taken place, but the size of the rock large on the lower slopes was considerably larger than that within the river gravel and cobble bars.

Post our traverse, a review of aerial photographs dating back to the 1930s found no discernible change in the position of the terrace edge over a period of 80 plus years. The historic aerials also showed the river was frequently flowing right at the base of the terrace. Hence, the rocks armoring the lower terrace were effectively reducing erosion for most river flow events.     

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