Granular ice observed on the Athabasca River near Fort McMurray, AB (Canada), January 2002
When our river ice field crew drilled this hole through the Athabasca River ice cover in January 2002, they were amazed to see it fill up with these hard little balls of ice. These pieces of ice looked just like marbles and appeared to be moving along the underside of the winter ice cover, just like sediment moves along a river bed. It was MSc student, Claudine Girouard, who came up with the most likely explanation for this. She suggested that this ice originated about 100 km upstream of Fort McMurray, in a rapids section that periodically opens up in winter. Ice forming in that open water area was likely swept under the downstream ice cover and broken into pieces in the process. Then, over its 100 km journey under the ice cover to Fort McMurray, these ice pieces became rounded – just like river rocks tumbling along a stream bed.
Over our 10+ years of conducting field work on the Athabasca River near Fort McMurray, this was the only time that ‘marble ice’ was ever observed.
(photo by Julia Blackburn or Kristel Unterschultz, UofA River Ice Group: Nikon E9500, f/6.6, ISO-80, FL 19 mm, 1/78 sec. exposure)