The first documentary evidence for use of the river occurs in the Anglo-Saxon Chronicle. It relates that early in the winter of 894 the Danes of Mersea rowed their ships up the Thames and the Lee. By the summer of the following year they had built a fortress twenty miles above London. King Alfred's forces attacked the fortress, but were put to flight with loss of life.
The retreating English force camped nearby in order to safeguard the corn harvest, at the same time denying it to the Danes.
Alfred then conceived the idea of obstructing the Lee to prevent the Danes from retreating in their boats. Saxon legend has it that Alfred drained the lower reaches of the river, cutting off the water channel at Waltham and stranding the enemy. Alfred also began to build two forts on either side of the river. Before construction had progressed far, the Danes abandoned their stronghold and their ships, sent their women to safety in East Anglia, and marched overland to Bridgenorth. The Vikings then scattered on foot and had to fight their way home.
Other details of the incident are later additions. Florence of Worcester implies that the river had been constricted by piers or dams and it was that other Norman writer, Henry of Huntingdon, who was the first to state that Alfred "caused the water of the Lee to divide into three branches, so that the Danes were unable to bring out their ships." Admittedly, at both Stratford and Waltham, the river divides into a number of streams but this is a natural phenomenon. The Danish fort must almost certainly have been built within the Danelaw, of which the Lee formed its south west boundary, so that they were well within their rights, but Alfred's activities may be seen as an effort to set up a demilitarised zone and also to discourage new Danish colonisation so near to London.
'The River Lea Campaign of 894-5' Herts. Past & Present No. 9. 1969 A.C.Jones, pp. 9-17