lea

The River Lea ( LEE) is in South East England. It originates in Bedfordshire, in the Chiltern Hills, and flows southeast through Hertfordshire, along the Essex border and into Greater London, to meet the River Thames at Bow Creek. It is one of the largest rivers in London and the easternmost major tributary of the Thames.
The river's significance as a major east–west barrier and boundary has tended to obscure its importance as north–south trade route. Below Hertford the river has since medieval times had alterations made to make it more 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, the 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 (high mineral content) of London tap water.

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