A stonefly nymph should have filamentous cercae and antennae. Maybe these have broken off giving it the appearance of a bug nymph. See pictures in the waterbug book.
@RogerF From Russell Yong who is one of the admins on the entemology group on Facebook:
It's a leptophlebiid mayfly nymph. Three posterior processes, blocky head, lateral serrations. Prob wandered above the waterline in wetter times and then got stranded.