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. Below Hemel Hempstead it runs alongside and sometimes forms part of the Grand Union Canal.