This is the alder spittle bug (Aphrophora alni) but it is actually associated with a wide range of deciduous trees and bushes, not just alder. Indeed, it is more likely to be seen in woodland rather than by rivers where the alder grows. The alder might be misleading but the spittle bug is not! This is one of the froghopper bugs that produces 'cuckoo-spit' to house its eggs and hatched larvae.
Large for a froghopper but still very small, just a centimetre long at most, it can be quite variable in colouring. The reasons for this are not known but one thing remains constant and that is the distinctive pale patches on the margins of the wings.
A common species but not often seen as most of its work is done at night. During the day it rests on the leaves of trees and shrubs. It is active from May until October but only produces one brood of off-spring each year.