From the Wikipedia page [1]
The River Gade is a river running almost entirely though Hertfordshire. It rises from a spring in the chalk of the Chiltern Hills at Dagnall, Buckinghamshire and flows through Hemel Hempstead, Kings Langley and Croxley Green to Rickmansworth where it joins the The River Colne. For its whole course the Gade is unnavigable.
Its principal tributary is the River Bulbourne which joins it at Two Waters, just below Hemel Hempstead.
The river was once used to power water mills at Water End, Cassiobury Park and Two Waters as well as powering the John Dickinson paper mills at Apsley and Croxley.
It supported the farming of watercress at Cassiobury Park, Water End, the Water Gardens and Two Waters until water was diverted from the river in 1947 to supply the growing new town of Hemel Hempstead.
Below Hemel Hempstead it runs alongside and sometimes forms part of the Grand Union Canal.
The remains of a Roman villa were found at Gadebridge Park in Hemel Hempstead.
The Gade is one of the rivers referred to in the name of the Three Rivers district.
The Chilterns AONB page is [2]