Stand knee-deep in a river and consider the energy flow. Water presses against you, light reflects upon the surface. What else can you feel? Helen Czerski of University College London views the Thames with the eyes of a physicist. At low tide she takes Eleanor Rosamund Barraclough for a wade through the surprisingly clean waters separating Brentford from Kew Gardens.

How would the river look if embankments were removed and channels breached? Why do islands form and persist? Where does each drop of Thames water come from? Why can the river flow east and west at the same time?

You may never view the Thames in quite the same way again.

Producer: Alasdair Cross