Well baza, I dunno. I live in Bishops Stortford and we have a great big, big motte right by the river in the valley bottom - not on the high valley ridges all around. It is Waytemore Castle and is generally reckoned to have been built on top of earlier fortifications - Saxon and posssibly earlier still. The area was marshy and flooded - so the motte sticks up out of the flood plain. Roman settlement was further up the river where Stane Street crossed it and then the Saxons did there usual thing of locating their settlement away from Roman ruins. The mottte is near to the ford which was named after a local Saxon gent called Steort who gave his name to the town and the river - Steort's ford. Long way of saying that your arguement aginst mottes being built on low land near rivers doesn't hold water (pardon the pun) ;D