Walking along the River Tees, I saw a logjam of flotsam penned in against the quay. Amongst the debris of plastic bottles, bobbing burger boxes and Styrofoam tea cups, a willow was floating. In spring, the council cleared some scrub upstream, and one of the felled trees, a crack willow, had floated down here to Stockton. After months immersed in the water, it had grown springy branches and long, slender, finger-like leaves.