"The River Why" was a life-changing book of philosophy and introspection for me. The book is subtly powerful and I highly recommend it. Such a book requires a journey that takes significant internal time. The book lets you wander comfortably for ages through beauty, ideas, love, and fish -- never realizing until it happens that these wonders are slowly and gradually coalescing into something tangible inside the reader.
There's no way for a movie to do the same thing. And so, of course, this film does not accomplish what the book does.
However, it is true to the book in its own way. For one who has read the book, the movie will take you back into it and help you reawaken that feeling inside. It does this by not trying to to it all, but rather, through music, scenery, and pace, reestablishing the feel.
For one who has not read the book, it's likely to be an enjoyable little glimpse into a tiny piece of a much larger river. Sit down and relax with a cup of tea or a glass of wine and let the movie happen.