2019 is turning out to be a very… different year. Above average snowfall, late snowfalls, high water runoff, delayed runoff; and now an avalanche hazard to take into consideration. On June 4th we had our annual staff training up in Rock Creek Canyon and found evidence of recent avalanche activity. The type of avalanches we saw were “wet slabs” and of up to a D3 plus size, on North and East facing aspects at 11,500 feet. With concern for backcountry travelers, we are offering a few warnings and pieces of advice, but first, a little more about D3 plus wet slab.

D3 wet slab avalanches from Mt Starr, Rock Creek Canyon.

D3 wet slab avalanches from Mt Starr, Rock Creek Canyon. Multiple failures, probably simultaneously of snow over an area of about 4-500 yards sliding about 1000 feet and leaving large fan shaped deposits of wet heavy snow below the cliffs. You might think you would be safe at the bottom, but whatever comes from above will fall on you.

D2 wet slab avalanche above Rock Creek Canyon. These start as a cohesive slab separating from the snow above leaving a distinct “crown fracture”, a step in the snow. There is a small cornice at the top with small loose wet avalanches starting just below it.

Avalanche Canada (2010) states “Wet slab avalanches are caused by a thick cohesive slab of snow losing its bond to an underlying thin weaker layer or interface after becoming damp, moist, or saturated with water. Wet slab avalanches are generally slower moving that dryer slab types, tend to flow in channels, and are easily deflected by irregularities in the terrain. Wet slabs are often highly destructive due to great mass created by the high water content of the snow.”

The D scale rates the destructive potential of an avalanche. According to American Avalanche Association (2016), a D3 avalanche “Could bury and destroy a car, damage a truck, destroy a wood frame house, or break trees.” The “plus” on D3 means the upper end of the rating. For perspective, the smaller D2 rating is defined as being enough to “kill, bury or injure a backcountry traveler.”

Learning about avalanches is a life-long avocation, and they are difficult to predict and evaluate. Just keep in mind:

  • Wet slab avalanches will most likely occur as temperatures increase and as solar radiation warms a slope.
  • Most avalanches start on 30-35 degree slopes with 37º being the most optimal angle. But avalanches can run out onto much lower angled slopes so one does not have to be on a steep slope to be buried. Having a steep slope above you can be as much of a danger as being on the slope.
  • Wet slab is not the only avalanche type to be concerned about. There are also wet loose avalanches that will happen as individual snow grains lose their cohesion and come down as a wet mass. Then there are glide avalanches where the whole winter’s snow mass slides down on the terrain underneath. This is a particular problem where the underlying terrain is a smooth rock slab – such as high Sierra granite.
  • There are cornices to consider. These are overhanging pillows of snow on ridges and passes, formed by wind over the winter. Every cornice has its day and will fall. Don’t be there when it does.

What to do?

  • Be aware of the potential for avalanches even though it is no longer the winter season.
  • Be aware of the effect of the sun and temperature as the day progresses. If you are feeling hot, so too is the snow.
  • Travel up and across steep slopes and passes early in the day before the snow gets wet and gloopy.
  • Move quickly across slopes and from underneath slopes without delays and stopping.
  • Look for wide cracks developing in the snow on steeper slopes. These could be indicators of a glide avalanche getting ready to happen.
  • A cornice can break a long way back from its lip so just stay away. Do whatever you can to avoid being underneath cornices.
  • Just because someone crossed a slope before you do not get lulled into a false sense of security. Maybe they just got lucky.
  • If you don’t like it then don’t do it. Set up camp and wait for cooler temperatures. Or reassess your trip and decide if it is worth the risk. The trail and the mountains will always be there; make sure you are around to enjoy them.­­­

Be safe, have fun.

References

Haegeli, P., Atkins, R., and Klassen, K., 2010. Auxiliary material for Decision making in avalanche terrain: a field book for winter backcountry users. Avalanche Canada, Revelstoke, B.C. Accessed at www.avalanche.ca.
American Avalanche Association, 2016. Snow, Weather, and Avalanches: Observation guidelines for Avalanche Programs in the United States (3rd ed). Victor, ID