The Mouth of the River was dated 1876/1877 in the 1995 catalogue raisonné, being dated by comparison with Study for 'The Mouth of the River' [M.0589], which in its turn was dated mostly from the butterfly signature. 1 Dating by butterfly is not an exact science, and the monogram varies according to the medium; furthermore there are few watercolours from this period, and when Whistler was in Venice (late 1879 to 1880) he painted hardly any. Researchers at the Freer Gallery of Art suggest a later date, between 1881 and 1883. Again, there are few securely dated works with which to compare it, with the added complication that some watercolours painted in 1881, such as Beach scene with rocks [M.0855], were probably signed later. However, one, The Sea - Grey and Silver [M.0858], probably dating from 1881, has a marginally comparable butterfly; ones of 1882-1883 appear to be different in detail. Thus it is possible that a wider date range, of 1876/1881, should be considered.

The Mouth of the River, Freer Gallery of Art