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.
It can be very difficult to separate H. fluviatile and H. tenax, and intermediates do occur (which may be recorded as Hygroamblystegium sp.). H. fluviatile has a wider and more ovate leaf shape, the widest part of the leaf tending to be higher than in H. tenax. It has an obtuse apex, and the cells of the apex are usually not much different in shape from the upper laminal cells (tending to be longer and narrower in H. tenax).Read the Field Guide account