The adult has an unstreaked grey-brown back, whitish grey underparts, and a darker undertail, which has white feather tips giving a contrasting pattern.

Some birds can show reduced dark markings on the undertail coverts (caused by more extensive than usual white tips) and thus are closer in appearance to Savi's warbler than typical river warblers.

It is similar to the song of other species in the group, but has more of a sewing machine quality, and may be produced for long periods.

This small passerine bird is a species found in dense deciduous vegetation close to water in bogs or near a river.

In Britain, a small number of males have set up territories in spring, including a bird in Greater Manchester in 1995.