Claude invented a pictorial format for landscape which was so satisfactory that it dominated landscape painting right down to the time of Turner and Constable, both of whom embraced and then went beyond it. Basically it involved balancing groups of trees (or buildings) in the foreground on the left and right of the picture; these act as 'repoussoirs' to stop you looking into the picture at the edges and guide your gaze to the centre, which then contains a carefully organised recession to a distant horizon. Turner's principal interest in 'Crossing the Brook' seems to be to recreate in a very grand and poetic way the delicate pale light and atmosphere of an English summer's day. This seems to be confirmed by his comment to a friend visiting his studio who noticed that a piece of paint had flaked off the picture: 'What does it matter', said Turner 'the only use of the thing is to recall the impression'.
The two girls are said to be Turner's daughters Evelina and Georgiana.
Simon Wilson, Tate Gallery: An Illustrated Companion, Tate Gallery, London, revised edition 1991, p.56