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 ( http://www.arlis.org/docs/vol2/hydropower/SUS102.pdf ). 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 18^th. 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 http://yukonriverpanel.com/salmon/wp-content/uploads/2011/04/ure-25n-10-yukon-temp-monitoring-on-yukon-trib.pdf )

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.