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.