• Andy@DueWestAnglers

Alpine Ice-Off: Timing the Thaw

Updated: Apr 14

interactive map: CO

interactive map: Rockies

As winter thaws, it can be difficult to hold back the cabin fever. Many anxiously await the opportunity to lace up their hiking boots and hit the trail for alpine trout, and for good reason. As alpine lakes (elevation > 10,000 ft) begin to shed their icy winter coats, trout move to the warmest water around the banks of these lakes to begin their feeding blitz. But what if you show up and the majority of the lake’s surface is still iced over? When can you expect your favorite lake to ice-off or be snow-free? We're here to help! In this article, we look at the data and help you time your early season outing into the alpine.



Predicting First Snow-Free Date

I’ll do my best to keep this short and painless. NASA has published data from the MODIS satellite, which has been collecting snow cover imagery since 2000. By the power of coffee and sheer willpower, we’ve distilled that information to relate elevation to the first snow-free day for every lake in Colorado. (Sorry for the Colorado focus—we can’t help it, we live here—but we did open this up to the other Rocky Mountain states also so keep reading!) I think this concept is pretty easy to understand; it's colder at higher elevations later in the year, and as you keep ascending, it takes longer to warm up. So, by no coincidence, lakes higher in altitude take longer to ice off than lakes at lower elevations. Still, this is an important calibration to confirm. Comparing our results to the Fly Fishing Guide to Rocky Mountain National Park: by Steven Schweitzer, we’re both in agreement about the dates and trends for lake ice off (see Appendix). We’ve just expanded his findings beyond Rocky Mountain National Park (RMNP) to be more useful to anglers fishing elsewhere. The below figures shows median ice-free dates for a range of elevations. These data were collected over twenty years between 2000-2020. The median date is represented in the box and whisker plot as well as in the summary table below. In the left figure, the top and bottom of each box represent the range of dates between the 25th and 75th percentile. This information is also distilled into a convenient table (right).

First Snow Free Day 2000-2020. Interquartile range and median are represented in the box and whisker plot.

If you already know where you're heading this spring, this should get you started. If you don't know where you're headed yet, keep reading and check out our interactive maps below. But why stop here? We had a few more questions to investigate.


Are we seeing ice-off happen earlier?

We were also interested to see how 2000-2010 snow-free days compared to 2011-2020 snow-free days. Days are counted from the start of each year and referred to as day-of-year (DOY), between 1 (January 1) and 365 (December 31). With a high degree of confidence, we found that for elevations between ~7500’-14500’ the mean snow-free day shifted from 124.37 (about May 5th) to 121.75 (about May 2nd). Though alpine lakes have been less impacted thus far, 2000-2010 mean: 153.15 (about June 3rd), 2011-2020 mean: 153.4 (still June 3rd).


How does El Nino vs La Nina affect ice-off?

The other big question we wanted to investigate was how seasonal shifts in El Niño vs La Niña years might influence an alpine lake’s first snow-free day. If you knew it was an El Niño year, would that change when to expect the first snow-free day? In Colorado, the effects are stronger between November and March (not so much during the spring/summer months). El Niño tends to push temperatures to below average. Some might think that El Niño also corresponds with the large blizzards from years past, but when we dug into the Colorado Climate Center's website, we found that they don't associate El Niño with these events. La Niña tends to push temperatures above average on the eastern plains in both winter and summer. Surprisingly, when we compared the ice-off day for high elevation lakes in El Niño years vs La Niña years, we found a significant difference where La Niña years were snow-free much later, 131.86 (about May 13th), while El Niño years were snow-free much earlier 114.84 (about April 25th). For alpine lakes greater than 10,000' the same effect is seen: El Nino years' first snow-free day: 146.8 (about May 28th) vs La Nina years' first snow-free day: 159.8 (about June 10th). This was a surprising result!



How does lake depth factor in?

What about lake surface area or depth? Lakes not only warm from sun exposure but also from the collective warming of the surrounding earth. As ambient ground temperatures increase in spring, the ground has a warming effect on high lakes. But, we found no relationship between alpine lake size and the first-day snow-free. However, it seems reasonable that smaller, shallower lakes will warm up faster than larger deeper lakes.


Limitations

We are making the assumption that snow-free and ice-off dates are directly connected and we've referred to them interchangeably here because they look similar in satellite imagery. I should also note that we didn’t exclude man-made dams from this investigation, nor did we look for only wilderness lakes. We intentionally kept this as broad as possible to gather as much data as we could to help draw meaningful conclusions. The data is noisier at times which we attribute to these key elements: 1) man-made or aerated lakes may become snow-free for unnatural reasons; 2) the closer a lake is to a road, the more likely it will appear snow-free earlier, as road crews maintain many roads throughout the winter and aren't able to be differentiated from imagery. Nonetheless, this was intended to be a useful guide for the high alpine lakes.


These findings are based on the best available data but they still rely on distant observations. There are no doubt local, seasonal, or yearly variations that might affect when lakes actually shed their winter ice. Plus, winter-kill events might drastically disrupt even the most perfectly timed hike. (Winter kills are most likely to occur in shallow lakes that become oxygen-deprived under the ice, and can be common for some alpine lakes.) With that disclaimer out of the way, we offer this as a data-driven starting point and remind you that there is no other fly fishing source willing or able to give you this level of detail.


An Interactive Guide

We’ve compiled the first-day snow-free data into interactive maps which you can use to explore your favorite lakes and figure out when they are expected to ice off in concert with the information provided here. By clicking anywhere on the map you'll see the 20-year average snow-free date for that specific area. Pair that with the information from this article to fine-tune your plan.


See the appendix for more details on using this interactive map. I encourage you to follow the link (Colorado or Intermountain West) to open the interactive map in a new browser on your desktop, but they are embedded here as well. [Hint: darker purple = later ice-off date]



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References:

  1. A. H. Armstrong. 2020. Identifying Annual First Day of No Snow Cover. Google Earth Engine Community Tutorials. https://developers.google.com/earth-engine/tutorials/community/identifying-first-day-no-snow

  2. J. Null. 2021. El Niño and La Niña Years and Intensities. https://ggweather.com/enso/oni.htm

  3. Colorado Climate Center. 2021. ENSO and Colorado. https://climate.colostate.edu/co_enso.html

  4. S. Schweitzer. 2011. A Fly Fishing Guide to Rocky Mountain National Park. Pixachrome Publishing.



Appendix:


Our trend-line vs Schweitzer's trend-line

Analysis shows similar results with previous work by Schweitzer

Web map Guide

The greater Rockies map is a bit coarser than the Colorado map. If you're interested in a more detailed looked at Utah (Unitas) or Yellowstone or anywhere else, let us know and we can crunch the numbers.


If you're not familiar with using interactive maps, there is functionality to view satellite imagery or topographic basemaps, change the opacity of the Snow Free layer, click on the Snow Free Layer to determine which Day of Year (DOY) is predicted, and search for your favorite locations by name or coordinate. Give it a second to load—there are a lot of data. If you're not sure how to convert day of year (DOY), do this: Google ## days from Jan 1.


Methods

Are you a real DWA Science nerd? Want to know more about how we did this? Check out our flow chart below. You're not gonna get this on other fly fishing blogs... Still not enough? Send us an email, we should talk!