Tuesday, April 28, 2020

Tripod Out

The Nenana tripod went out yesterday, marking the break-up of the Tanana River at its confluence with the Nenana River.  As I noted last week, it's highly likely that the break-up was earlier than it would have been without such abundant run-off from excessive late winter snowfall across the region.

This claim is supported by the fact that this year's break-up occurred with the least number of thawing degree days (TDDs) on record in nearby Fairbanks: the total TDDs through yesterday in Fairbanks was 74.0, which narrowly beats out 2002 (74.5) and 2006 (75.0).  We might say this year's thaw season leading up to break-up was the coolest on record.

Here's an updated chart of the relationship between TDDs and precipitation amount from March 1 to break-up.

An interesting nuance here is that most of the years with high precipitation and low TDDs also saw relatively late break-up, in contrast to this year.  The chart below classifies the years as "early" or "late" simply based on whether break-up was before or after the long-term median of May 3 (and yes, there's a significant trend).

It seems that it's typical for wet years to also be cool in April, and this delays break-up in comparison to dry years; i.e. dry and warm tend to occur together in late winter and early spring.  This complicates the picture a bit, because the stronger sunshine at later dates allows late break-ups to occur with lower TDDs independently of precipitation effects.  So the precipitation/TDD relationship isn't quite as simple as "more run-off means earlier break-up"; the relationships are complex.

A final bit of chart analysis (see below) illustrates that the very latest break-ups reliably occur after low TDD totals; but this year was unusually early for such low TDDs.  I suggest this is almost certainly because of the increased run-off related to the wet conditions of the past 6 weeks.


  1. Big plug of snow/ice melt water flowed from upriver tributaries prior to the tripod event



  2. Interesting information. I was surprised by how early the tripod went out this year, considering how severe our winter was and how cool April 2020 has been. I live on a creek in the Alaska Range, North side, which flows into the Nenana River. Once my creek starts to flow, it usually is about ten days until the tripod goes out.i have been noting for the last five years or so the conditions that prevail which lead to my creek flowing. This April has been rather cool, with high temps in low to mid-forties only leading up to the start of my creek flowing on the 18th. What I did note, which may be a significant factor, was the presence of constant Chinook winds, which I believe increased the melting of the snow, even at the cooler temps. This may have been a compensatory factor worth investigating in any theories correlating late season snowfall and cooler April temperatures.

    1. Hi, thanks for reading and for the observations. Yes, there's no doubt that wind makes a tremendous difference for snow melt rates, so could be quite an important factor in break-up timing. Historical wind data are problematic, but it would be an interesting topic to explore.