 Competition for the use of the river created the centuries-old conflict between those who wanted to dam the river to build millraces and fish traps and those who wanted to travel and carry goods on it.Economic prosperity and the foundation of wealthy monasteries by the Anglo-Saxons attracted unwelcome visitors and by around AD 870 the Vikings were sweeping up the Thames on the tide and creating havoc as in their destruction of Chertsey Abbey.Once King William had won total control of the strategically important Thames Valley, he went on to invade the rest of England.