This is a scarce liverwort but in favourable sites it can be abundant. It is a member of a very special assemblage of river-associated species that also includes (depending on location) Dendrocryphaea lamyana, Fissidens rivularis, F. polyphyllus, F. serrulatus and Rhynchostegium alopecuroides. More common associates may include Rhynchostegium riparioides, Racomitrium aciculare, Thamnobryum alopecurum and Sciuro-hypnum plumosum.
Look for Porella pinnata in fast-flowing watercourses of circumneutral reaction. It grows in and out of the water but is never very far from the waterline. Large populations are usually striking (unless the river is very high!) and form a black ‘tide-mark’ on rocks and exposed roots. Smaller populations may not reveal themselves so easily, but can sometimes be winkled out of hiding in places that are protected from the strongest currents, such as on the submerged root-plates of alders and on the lee-side of regularly inundated rocks.
The lobules and underleaves are very small for a Porella and you may struggle to see them with a hand-lens, especially if the plants are silt-encrusted.Read the Field Guide account