This is usually an easily recognised species, as it is the commoner of two species of Orthotrichum that are riparian specialists. Both also have obviously obtuse leaf apices which sets them apart from nearly all of their relatives, except Nyholmiella gymnostoma and N. obtusifolia, which are rare epiphytes of dry places. O. rivulare is usually blackish in appearance and is frequently fertile.
In certain limestone districts, a darkly coloured riparian form of O. cupulatum (formerly named var. riparium) grows by fast-flowing rivers and resembles O. rivulare. However, close examination of its leaves will show that they have an acute, not obtuse tip.
It is often necessary to removed silt encrusting plants before microscopic examination and this can be done either by shaking whole plants vigorously in water in a small capped container, or by using a fine paintbrush to stroke off the silt when the plant is immersed in water on a slide.
O. rivulare grows on rocks, tree roots and bark in the inundation zone of silty rivers. It dislikes base-rich habitats and is absent from most of south and south-eastern England. It is distinguished from the scarcer O. sprucei by size (it is a larger, more sprawling plant) and smaller mid-leaf cells. O. sprucei has less catholic habitat preferences and is nearly always found on roots or bark of trees low in the inundation zone of large silty rivers.Read the Field Guide account