British distribution: An evident introduction, now frequent north to at least the south of Scotland.
Another Agrocybe that has recently appeared in Britain on wood-chips is A. putaminum. First recorded at Kew in 1986, it appeared in Manchester in 1996 and now appears to be increasing in southern England. It differs from A. rivulosa in lacking a ring, and the cap is described as having the texture of chamois leather. Both species are included in Buczacki et al. (2012), though the illustration purporting to be A. rivulosa seems more likely to represent A. putaminum.
Correct identification becomes essential if A. rivulosa is eaten. We have no folk lore history of experience of this species, no information on whether people may react to it in different ways, yet several websites claim it is edible — though there are French websites that state, "sa comestibilité est sans intérêt." The possible trap I have not seen mentioned is the potential for confusion with dangerously and fatally poisonous Amanita species. Some of the first Internet photographs of A. rivulosa were first identified as Death Cap (Amanita phalloides) and confusion could happen in the opposite direction! My experience with Internet identification forums is that people with very little knowledge can make completely confident but utterly ridiculous identifications — indeed "identification" apparently consists of finding a superficially similar Internet photograph, without any understanding of key characters and not reading any accompanying text. Volval characters in Amanita are variable and I have had one person belligerently adamant that his photograph was not an Amanita, and had no ring or volva, when remains of both were clearly visible in the photograph. In A. phalloides, the volva is fragile and can soon fall apart.
Now, Dear Reader, obviously you, yourself, are more intelligent than this, but imagine A. rivulosa is being collected along a wood-chip path. It has been carefully checked, including the brown spore-print, but overlooked has been one fruitbody that was actually a brownish variant of the Death Cap, or maybe a discoloured Destroying Angel (Amanita virosa), that has pushed up through the wood-chips from soil and tree roots below. It only takes that one fruitbody to turn a meal into a tragedy.