Thursday, April 9, 2015

Sag River Saga

In the past several weeks a saga of Sisyphean struggle has unfolded for Alaska DOT workers about 15 miles south of Deadhorse, as overflow from the Sagavanirktok River has repeatedly closed the Dalton Highway.  I started wondering if weather conditions of recent months could be responsible, so I pulled up some data.  First, it's been a (relatively) very warm winter in the area; Deadhorse airport recoded the second highest November-March mean temperature of record when compared to the combined Deadhorse/Prudhoe Bay history since 1969 (see below).

Temperatures have been consistently above normal, with every month since October falling in the top third of the historical distribution.

As for precipitation, data from SNOTEL sites along the Dalton Highway reveal that the water year ending September 30, 2014, was wetter than the long-term normal.  At the Sagwon SNOTEL, about 50 miles south of Deadhorse, 2014 was close to the wettest year on record, although the anomaly was not as great at either Imnaviat Creek (closer to the Brooks Range) or Prudhoe Bay.  Note that some years have missing data in the chart below.

Could the weather anomalies explain the massive overflow south of Deadhorse this year?  I think it's possible that the combination of high precipitation last year and a very mild winter have allowed the Sag River streamflow to remain higher than normal (under the surface ice and above the permafrost) even at this time of seasonal minimum streamflow.  The Sag River streamflow gage near Pump Station 3 hasn't reported in recent months, so I can't confirm this, but it seems physically reasonable.  With the winter's cold still being sufficient to freeze the river to the bottom in places and cause backups, the excess volume of water has become apparent in the widespread overflow.

Of course it's also possible that this event is a random occurrence related to an unusual configuration of ice formation.  However, if the weather did play a role, then we might see more of this in future years if reduced Arctic sea ice continues to produce a wetter, warmer climate on the North Slope.


  1. Disaster area w/pics:

    An Internet search for "Aufeis" will turn up many citations and analyses as to their formation. In the few years I accompanied others investigating overwintering fish habitat in association with Aufeis Fields on the North Slope, I was always amazed at their expanse and the depth of the ice.

    We'd drill 8' or more of solid ice to reach a few feet of running water (and hopefully fish) beneath. We first used previously implanted radio transmitters to locate their general location. The fish were normally located upstream of that portion of the Aufeis fields that froze to the bottom.

    If the under-ice downstream flow gets blocked, the fluid water upstream simply escapes over the top if the ice cover (eventually increasing the ice thickness), or sideways out of the normal constraints of the channel(s) as is apparently happening this year on the Dalton Highway.

    Richard's comments appear to very adequately address this year's extreme situation.


    1. Thanks Gary - very interesting. To me, the surprising aspect is the relatively large volume of water flowing under the ice in winter. Evidently groundwater flows are substantial year-round even in continuous permafrost areas.

    2. If one flies across the North Slope of Alaska and Canada E<>W in winter those Aufeis fields are visible to some degree on every north flowing drainage. It's not a matter of if but where they eventually develop prior to reaching the coast.

      Groundwater upwelling at declining elevations periodically contributes to the water volume and flow. Given the mountains to the south and porous gravels you're right about the potential for substantial flows of water in the Thalweg channel above underlying permafrost.

      Stream bottom elevations rise and fall and form a discontinuous series of relatively deep pools interspaced with shallow (riffle) areas. I assume the shallow areas permit freezing of the water column top to bottom during intense cold, blocking the normal channel thus forcing the unfrozen water upstream to seek new passage.

      Compounding the blockage problem are the presence of bottom-forming anchor ice and floating frazil ice crystals in super cooled water. Both can eventually build a hollow core of flowing river water entrained in solid ice.

      As the Aufeis fields form it's typical to have several feet of overburden ice, then an air space, then a layer of running water on top of bottom gravels. Fish know all about this of course and spend the winter hopefully swimming in safe spots waiting for Summer's thaw.