Hi Gary,
I saw this myself last week. Don't think that the Dee was frozen, more that the ice/grue picked by the river is carried down to the tidal stretch from Leggart Terrace to the harbour. With the rise and fall effect of the tide, this is deposited on the banks and builds up. The island at the "Pots and Fords" stretch , near the Ghillies Lair, was covered in pure white ice to a depth of two to three feet , looked absolutely beautiful, but at 2+ feet thick, most unlikely to have been the Dee itself freezing to that depth.
Lyall