I bought this film after having enjoyed a (subsequent) film by director Etienne Chatieliez, 'Le Bonheur Est Dans Le Pre' . This was a warm and funny film about someone who assumes another's identity, and so changes their life.
"Life is a Long Quiet River" also deals with swopped identities, but after the initial denoument there's very little plot movement, and the whole thing is thin on the ground and far too long.

I found "Life is a Long Quiet River" a real disappointment- I strongly advise you to watch 'Le Bonheur' instead, it's so much better!