The Chelmer and Blackwater Navigation is the canalisation of the Rivers Chelmer and Blackwater in Essex, in the east of England. The navigation runs for 13.75 miles (22.13 km) from Springfield Basin in Chelmsford to the sea lock at Heybridge Basin near Maldon. Initial plans faced spirited opposition from Maldon, which were overcome by avoiding the town and terminating at Heybridge, and the navigation opened in 1797. There were some teething problems, and the engineer John Rennie was called back on two occasions to recommend improvements. The impact of the railways was less severe than on many canals, as there was never a direct line between Chelmsford and Maldon. The sea lock at Heybridge was enlarged after the Second World War, but trade gradually declined and ceased in 1972.