Evidently the magic clinging to the house extends to the river, because the three children have numerous magical experiences -- from flying horses to encounters with a giant. Oskar shrinks to the size of a field mouse, they meet a hermit, and go back in time to a moonlight ceremony older even than the ancient house itself. But there is also the everyday magic of swimming, rowing, picnicking, and shooting through the locks in flood.
As a child, I didn't read The River as often as the other books, being at heart in indoor rather than an outdoor person, but I enjoyed it this time more than I expected. The friendship between the three children is lovely. 'There isn't anything real except thoughts,' says Oskar, so I suppose the magic in this book might be created by the children themselves, just as they create the long river map on a roll of wallpaper. But that's a very grown up attitude, just like Dr Maud Biggins, who refuses to recognise a giant even when she sees him with her own eyes, so I think I'll side with the children instead and say it's all real.