H. fluviatile can be recognised as a Hygroamblystegium by the combination of a very stout nerve and thick-walled mid-leaf cells only 2-4 times as long as wide. With H. tenax, it shares these characters with Cratoneuron filicinum, another highly variable species that is often found in highly basic wet places (although rarely on rocks in fast-flowing watercourses that are regularly inundated). However, that plant has inflated alar cells forming obvious decurrent auricles, which are lacking in Hygroamblystegium species.
When growing on rocks in fast-flowing water, the long, little-branched shoots of H. fluviatile have a characteristically stringy look about them but they vary in size and both robust and slender forms can be found. Water shear and abrasion sometimes erodes the laminae of the lower leaves, leaving only nerves sticking out from the stem.Read the Field Guide account