Lee Valley online
Verger's House, Hertford
The River Lee runs from Luton to the River Thames, and for much of its course runs through Hertfordshire. This site includes a history section with information on some of the Hertfordshire Towns it runs through. The following is an extract:
"During the 9th century the river formed part of the boundary between Saxon England to the west and the invading Vikings to the east, led by King Alfred the Great and Guthrum respectively. It is said that when the Danes sailed up the river in around 895AD and established a base near Ware, Alfred stranded them there by reducing the level of the river. He did this by building an embankment and weir near the Thames, and dividing the river just above Waltham Abbey. How true this is we do not know, but the remains of Viking ships have been found near Hertford and Stanstead Abbots, the river does split into three near Waltham Abbey and the names Kings Weir and Blackwall date back to these times."