I am not sure when this picture was taken, probably after the full flood, but the beck width on the old bridge sets the width required for the river channel to cope with the flood water. The water must be 2.5m deep at least, but the water is probably backing up from a restriction below. If the river channel has any less cross section to the top of the bank it will overflow and cause flooding. This may be acceptable in farmland but the town should be protected.
Please remember this picture when veiwing all the other photographs - this may be the key to the floods in Pickering?