Saturday, October 17, 2015

Yukon River at Eagle Ice Running

Richard noted in the previous post that ice was running down the Yukon River in Eagle. Is this typical? What drives the formation if ice in interior rivers?

Let’s deal with the questions in reverse order. Obviously you need below frezing temperatures for ice to form. But, it is not as simple as that. The drivers of river water temperature are very complicated. First, the top layer of water on a river needs to be at or slightly above freezing. This is largely driven by basin-wide characteristics of flow, air temperature, and the shortwave/longwave energy budget. The water flowing past Eagle mostly came from the Canadian Yukon Territory. There are nearly 1,100 river miles of the Yukon above Eagle and a basin area of 122,000 square miles. High flows mean greater stream velocities which delays freezup. High air temperatures also delay freezup. Especially cold temperatures early in the season have to overcome the still (relatively) strong solar radiation. All in all it is a complicated process. A nice summary of the processes is found in this document from UAF ( ). The document is more concerned with water temperature though.

For rivers like the Yukon, ice formation is initially dominated by the frazil ice process. Frazil ice is formed when supercooled water freezes on a condensation nuclei within the moving water. This generally happens when the low temperatures is in the lower 20s. As more and more frazil ice forms in the main channel, it clumps together – finally clumping enough to accumulate at river bends and to ultimately run across the width or the river.

The Cooperative observers in Eagle kindly made a notation of the first run of ice they saw for 22 different years between 1916 and 1960. During those years, the average first run date was October 18th. We should caution that a little bit of ice one year may be noted while another year required more substantial ice to warrant a notation. We will never know what the subjective criteria was. For this analysis, we assume they were consistent. Figures 1 and 2 show an example of notations made by the observer.

Figure 1. October 1920 Cooperative observer form for Eagle, Alaska.

 Figure 2. October 1947 Cooperative observer form for Eagle, Alaska.

Since frazil ice forms locally (i.e., doesn’t form in one spot and float for miles downstream), we expect that the formation is highly correlated to water temperature and air temperature. Unfortunately there are no good data sets for water temperature. Figure 3 shows the water temperature at Eagle in 2010. It’s the only data set that I found.

Figure 3. Water temperature for Yukon River at Eagle, Alaska. (from )

Since we are then left with air temperature as the best proxy for ice formation, let’s see how well it correlates to freezing degree days. Figure 4 shows the ice run date for the 22 years that data is available (line) and the accumulated freezing degree days (bars). There really is very little correlation. Some years saw ice form with as little as 5-6 days of temperatures in the 20s. Other years saw 4-5 days in a row with temperatures dropping below zero before ice began forming. Figure 5 shows the relationship between accumulated freezing degree days (FDDs) and the date of ice running. Not much there.

Figure 4. First ice running date and accumulated freezing degree days for 22 years at Eagle as lines and bars.

Figure 5. First ice running date and accumulated freezing degree days for 22 years at Eagle as scatter plot.

Again, so much goes into the formation of ice. Is it clear or cloudy? Is it windy? Is the air dry or moist? Is new runoff entering the river? Is the flow fast or slow? Is it high or low? Lots to consider.

As a non–interior Alaskan, I welcome the opinions of those more knowledgeable on the subject than I am.


  1. My obs. Clear (more radiation stuff) and calm helps initiate freezing, clouds and wind slow it down. Can't put a value on each.

    Did live next to the spring fed (39F year round) Delta Clearwater River. No ice mid-river going downstream until -40-50's air temps and clear in late 1975. Then it froze on the bottom first accompanied by the surface frazil and eventually the whole to bottom...except for an open hollow culvert-like channel between the surface and the bottom near the shore where I drilled to get water. Once it warmed to -20 or so it melted except for a thin intermittent surface covering. Did get to -63F that spell. Kept a thermometer in the outhouse as a reminder.


    1. Carlson's synopsis is good and it's a treat to return to simpler days when writers wrote to the audience and not just to themselves. The minimal use of citations goes contrary to many of today's papers that are mulched with bibliography in support of limited text. Some of that is a reflection of the author's direct experience in the subject.

      My feeling is that a variable volume of flow (river discharge volume over distance; (11, q) in the paper) over years has an effect on time of ice formation.

      In years of higher water volume (potentially due to previous precipitation and resulting higher water tables) the date of freeze may be later. A plot of estimated discharge versus freeze may reflect this if data are available.


    2. Maybe this would suffice as a proxy for volume if available:



      Sorry for all the posts...just peeling back the onion.


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