A very interesting looking rush/sedge, which appears to have a tree-like trunk. I couldn't get any closer to it because of the water and the blackberries on the shore.
Hi Michael, As you say, from the distance it is a bit tricky to be sure, but I think it is probably Carex polyantha. The 'trunk' will be an artefact of the soil being washed away around a very thick root mass, leaving it looking like a grass-tree. There are some old inflorescences that are branched, combined with the length of the leaves, C. polyantha would be likely, and I note that the Carex has been recorded nearby.
Thanks Russell, Your conclusion seems quite sound, so I've confirmed it as Carex polyantha. I will revisit next Spring when there should be some fresh inflorescences for further verification.