Having got to know all the animals on the river, Charlie, Philippa and the boys have lots of questions about them. This episode sees the family testing their theories and conducting a few home-made experiments.
Charlie believes otters can smell underwater and sets up his underwater cameras to test his theory; what he discovers could be new to science.
The family then goes off in search of elusive white-clawed crayfish and they find a few at a secret location known as an 'ark site'.
Charlie is keen to see how many fish it takes to feed a family of kingfishers in a season and sets up an experiment to find out.
And Fred and Charlie test out a super slow-motion camera to see if they can work out how many times dragonflies can beat their wings in a second - the results are quite amazing.
As the season draws to an end, Fred and Charlie get their closest encounter yet with a family of newly fledged kingfishers.