River Lea

The River Lea, also spelt Lee, is a river in South East England. It originates in the Bedfordshire part of the Chiltern Hills, and flows southeast through Hertfordshire and then Greater London, sometimes through several channels, to ultimately meet the River Thames, the last looping section being known as Bow Creek. It is one of the largest rivers in London and the easternmost major tributary of the Thames. Its valley creates a long chain of marshy ground along its lower length, much of which has been used for gravel and mineral extraction, reservoirs and industry. Below Hertford the river has since medieval times had alterations made to make it gradually navigable for boats between the Thames and eastern Hertfordshire and Essex, known as the Lee Navigation. This stimulated much industry along its banks. The navigable River Stort, a main tributary, joins it at Hoddesdon.

While the lower Lea remains somewhat polluted, its upper stretch and tributaries, classified as chalk streams, are a major source of drinking water for London. An artificial waterway known as the New River, opened in 1613, abstracts clean water away from the upper stretch of the river near Hertford for drinking, and lower parts of the river are also abstracted from. The Lea's origin in the Chilterns contributes to the extreme hardness of London tap water.

Wikipedia