While not the best in the series, this book is still enjoyable. The first half of the Adventure series is considered superior to the second half.
The Goodreads reviewer finds 'The River of Adventure' to be an enjoyable read, though not the best in the series. They note that the first half of the Adventure series is generally better than the second half, which includes this book. Despite this, the reviewer still appreciates the story for its adventure and mystery. They acknowledge that it's not bad at all, just not Blyton's finest work.
Quick quotes
So it's not bad at all, but for me this is not Blyton at her very best.
In my opinion the first half of the Adventure series is superior to the second half.
The story is enjoyable, though not the best in the series.