Long-time readers may recall a few posts last winter that mentioned the exceptionally slow freeze-up of the Yukon River in Dawson (Yukon Territory). It's interesting to note that the same thing has happened this winter - there is still a substantial gap of open water that has prevented the construction of the usual ice road to West Dawson. This year an effort was launched to accelerate the freeze-up with a "slush cannon", but lack of progress led to the project being called off recently.
Here's how the river looked last Sunday.
Last winter we noted that an unusual ice jam upstream was the primary reason for the open water, rather than exceptionally warm weather. It appears that the same may be true this year, because again the weather has not been particularly warm overall this winter in Dawson; the chart below shows that total freezing degree days through January 22 (when the ice-making project was canceled) were very close to the normal of the last 20 years.
The repetition of the slow freeze-up this winter suggests that it's not a random occurrence - "something" has changed - but the temperature data rules out a simple explanation based on excessive warmth. I started to look for river flow data from the autumn to see if the river discharge has been unusually high or low, but I had difficulty acquiring suitable data.
One possibility (and this is mere speculation, being outside my field of knowledge) is that there could have been a subtle change in the river profile/cross-section upstream of Dawson, owing to deposition or erosion, that may have altered the hydrodynamics in a way that now favors the formation of an ice jam in a new location. If any readers have comments on this idea, or other suggestions on how to identify the underlying cause of the open water (i.e. is it related to weather and climate), then I'd be glad to hear them.