The ice cover formed on Tágà Shäw (the Yukon River) at Dawson during the weekend of Nov 13-14. This is always of particular interest for Dawsonites and West Dawsonites, considering the partial freeze-up winters of 2013-14, 2016-17, 2017-18, and 2018-19.
Q: How does the ice cover formation sequence look this winter?
The Sentinel 2 Satellite image presented above was captured on Nov 11 and shows small ice pieces flowing down Tágà Shäw and being choked by the Tr’ondëk (Klondike River) delta (south yellow arrow), then by the narrowing of Tágà Shäw at Moosehide (north yellow arrow). You can see in the image above that loose ice coming from upstream (South, bottom of the image) is compressed and released in the form of large ice rafts that disintegrate in the next fast flowing section.
The following image from Nov 13 is actually a radar product from Sentinel 1. With radar imagery, the signal collecting the image “sees through the cloud coverage” and detects surfaces of different roughness (and the roughness of the newly formed ice cover is different from the roughness of water). It shows that the ice choking at Moosehide created a surface congestion (stationary ice from bank to bank, yellow arrow), with open water (in black) downstream (to the north) and light grey upstream (in front of Dawson, where some building roofs are bright white dots). A trained eye can even distinguish that the ice front (where ice coming from upstream stops at the water surface against the stationary ice cover) is located just upstream of the Tr’ondëk delta. This means that, on Nov. 13, the ice cover had just formed at Dawson and was therefore still fragile because not fully frozen.
The following image from Nov. 17 is also a radar product from Sentinel 1, but taken at a different angle, and using a different filter: The ice cover is pink, open water is black (downstream of Moosehide), and the cliff across the river in West Dawson is bright white. When you compare the Nov. 13 and Nov. 17 images, it can be noticed that the ice cover has shifted at Moosehide, and that there is an open water area along the left bank (West Dawson side) at the usual ice bridge location (yellow arrow) and extending downriver besides the Yukon Government campground. This narrow gap in the ice cover was visible on Nov. 14 on the webcam located above the Dawson City Firehall and it is still there as these lines are being written.
It is interesting to note that Moosehide was the first location, by far, to cause an ice congestion between Ts’ekínyäk Chú (Pelly River) mouth (far upstream of Dawson) and the Alaska border (far downstream). Even several days after this congestion occurred, no other ice bridging location had been detected along 400 km of river (there is another ice congestion location downstream of Eagle, Alaska). The ice front has been migrating upstream (intercepting all the ice produced in Tágà Shäw and its tributaries) and has now passed the White River mouth (Nov. 18-19). All this information is extracted from free Satellite products available online; such a great resource.
An initial ice congestion at Moosehide is usually a good sign because it normally translates into a complete ice cover in front of Dawson. The open water gap that formed between Nov. 13 and 14 in front of Dawson is different from what has been observed in previous problematic winters: it is another type of anomaly, it is located in a shallow area, and it may progressively form an ice cover on its own.
Q: Can we predict the timing of river ice formation?
The short answer is yes.
A simple model was developed in 2020 to determine the timing of river ice formation at Dawson (described here). This model relies on air temperature indicators making it very simple to implement. Two conditions are required to generate ice in this model. First, a minimum of 150 cumulative degree-days of freezing (CDDF, which is equivalent to 15 days at -10ºC or 10 days at -15ºC) is required for the whole river system to cool down and for enough stationary ice to form along the riverbanks (called border ice). Once this condition is reached, and that only the central (deepest and fastest) part of the channel remains open over several hundreds of kilometers, the model shows that a series of colds days is required, normally with average temperatures below -20ºC, for enough ice (frazil particles, brash ice, ice floes) to be produced in the moving water. A high surface concentration of drifting ice is needed to generate ice congestion, as observed at Moosehide a few days ago. There are very few locations along the river where the first congestion normally happens (we refer to these as “dominant congestion sites”).
The following figure presents model results for 9 consecutive falls, including 2021 in purple. Each annual data set begins at the bottom left corner on October 1 and each line extends towards the right and upward over time. Negative temperatures are cumulated, depending on the sequence of cold days, up to 150 CDDF (shown as a horizontal line), which normally happens in late October to mid-November. Diamonds on each series represent the days where the ice cover formed at Dawson (some years have multiple diamonds, indicating ice formation date uncertainty). River ice formation at Dawson may only happen once series cross the 150 CDDF line and if their respective steepness corresponds to days at -20ºC or colder. In 2021, these conditions were respectively achieved on Nov. 10 and on Nov. 11, and the model, although simple, seemed to perform adequately as the ice cover formed on Nov. 12-13 (two purple diamonds).
Q: Can be predict the way the river ice cover forms?
The short answer is “not accurately”, but here is what we know:
While developing a river ice formation pattern model for Tágà Shäw, we identified that an initial congestion at the Tr’ondëk delta was the reason why the ice cover was only partial at the ice bridge location (as observed in 2013-14 and from 2016-17 to 2018-19). The initial congestion location (Moosehide or Tr’ondëk delta) was tentatively linked to the discharge of Tágà Shäw in a first version of that model: At low flow, congestion would occur first at the Tr’ondëk delta whereas at high flow it would occur first at Moosehide, the “ideal” location. However, Nature proved to be more complex than this model proposed, and its performance was not consistent in 2019-2020. The open water area that currently remains at Dawson (as these lines are being written) is one example of the partial failure of the model to capture complex, and apparently chaotic, hydrological processes.
One of the reasons why we can’t reliably predict river ice formation patterns near Dawson using the discharge of Tágà Shäw is that this information is largely unknown at this time of year, precisely because of the dynamic formation of stationary ice that affects water level measurements from which flow is estimated. YukonU is currently working with the Water Survey of Canada to improve discharge estimates in the presence of ice.
Beyond flow, several factors can affect ice formation patterns in this river:
- Wind and cloud conditions (this affects heat loss at the water surface and may interfere with water velocities)
- Snowfalls (this adds ice to the flow)
- Air temperature fluctuations (significant variations happened in the fall of 2020, a very dynamic year with an ice bridge)
- Changes in channel morphology or geometry over time (this affects ice congestion processes)
These factors need to be taken into account, somehow, for a model to adequately simulate river ice formation patterns in Tágà Shäw. Satellite images, as presented above, will continue to be analyzed and a more reliable predictive model will be developed. In addition to foreseeing ice cover conditions, this model could be used to inform cost-effective approaches to support the development of an ice bridge between Dawson and West Dawson.