Cranefly!! Not sure if this is the exact species as the ones I saw were better contrasted the top of the worm being darker brown, with the faint white lines Which I assume are innards or whatever.
I thought cranefly initially but I did not, for some reason, expect them to be so big and indeed so abundant. There were plenty of these things around although they were pretty well hidden. their defense mechaism of haphazardly bending into a grubby croissant must work against them when trapped in current as they seemingly will tumble about in a crescent until current subsides and they are able to retreat into cover?
I think a size 4 or 6 caddis worm hook? favorite cranefly patterns anyone?