streams flow into the Thames, some large enough to have exerted a significant influence on the geography of the area. Many of the smaller London tributaries now flow underground. Left bank tributaries Larger left bank tributaries include the Colne, Crane, Brent, Lea (tidal reach known as 'Bow Creek'), Roding (tidal reach known as 'Barking Creek'), Rom (lower reaches known as the Beam) and Ingrebourne. There are many smaller, now often largely subterranean streams including Stamford Brook, Counter's Creek (also known as 'Chelsea Creek'), Westbourne, Tyburn, Tyburn Brook, Fleet and Walbrook. Some of the tributaries are themselves large enough to have named tributary streams, for example the Moselle, Salmons Brook and Pymmes Brook that feed the Lea, and the Silk Stream and Dollis Brook that feed the Brent. Larger rivers such as the Lea have influenced local geography in several ways: A river and its marshland formed a significant barrier to east–west movement: the Lea formed a natural boundary between the historic areas of Middlesex and Essex. The valley of the Lea formed a transport route, including the river and later Lee Navigation, as well as roads including the Roman Ermine Street, the Hertford Road (A1010) and the later Great Cambridge Road (A10) and A1055. The Lea Valley is also followed by two routes of what became the Great Eastern Railway, and had important marshalling yards and locomotive works at Temple Mills. The river provided power for numerous water mills, such as the Royal Small Arms Factory at Enfield and nearby Waltham Abbey Royal Gunpowder Mills, Wright's Flour Mill (London's last surviving working mill) at Ponders End and the Three Mills at Stratford. In the 19th century the lower Lea became an important area for the manufacture of chemicals, in part based on the supply of by-products such as sulphur and ammonia from the Gas Light and Coke Company's works at Bow Common. In the 20th century the combination of transport, wide expanses of flat land and electricity from riverside and canalside plants such as Brimsdown, Hackney, Bow and West Ham led to expansion of industries such as Enfield Rolling Mills and Enfield Cables, Thorn Electrical Industries, Belling, Glover and Main, MK Electric, Gestetner, JAP Industries, etc. Much industry has now gone, to be replaced by warehousing and retail parks. The valley also became very important for London's water supply, as the source of the water transported by the New River