The Rivulet (Perizoma affinitata) is a species of moth of the Geometridae family. It is found in most of Europe.
Its wingspan is 24–30 mm. is characterized by the narrow white postmedian band of the forewing and especially by the dark hindwing, with only a narrow, divided white or whitish band. N. and W. Germany, and rather less extreme from England and according to Staudinger it is distributed in Central and Northern Europe and Roumania. - rivinata Fisch.-Rossl. has the white on the forewing much extended and the hindwing broadly or almost wholly white. It belongs chiefly to northern or mountain districts, but sometimes occurs as an aberration with the type England, N. Norway, the Alps, Carpathians. - magistraria Trti. and Verity is larger, the forewing grey, not brown, but darker than in hydrata and with broader white band ; hindwing greyish with double whitish band; underside like that of rivinata. Terme di Valdieri, Maritime Alps. 
The larvae feed on Silene species, including Silene dioica. The larvae can be found from June to September. The species overwinters as a pupa.