Terrain traps: why a small slide kills and a big one does not

People assess avalanche risk mostly in terms of whether a slope will slide. That is half the question. The other half — the half that determines whether a slide is survivable — is what happens to the snow and to you once it starts moving.

This is the concept of the terrain trap, and it explains why a great many fatalities involve small avalanches on unremarkable slopes.

What a terrain trap is

Any feature that increases the consequence of being caught, usually by concentrating debris, by causing trauma, or by preventing escape.

Gullies and creek beds. The most common. Snow that would spread thin across an open face piles deep in a narrow channel. A slide that would bury you to the knees on a plain slope buries you completely in a gully, and the depth increases dramatically. Gullies also funnel you into the deepest part rather than letting you move sideways off the moving slab.

Trees. Trauma is a substantial cause of avalanche deaths, and being carried into timber at speed causes it. Even a modest slide moving through spaced trees is extremely dangerous.

Cliffs and drops. A small sluff that would be trivial on an open slope becomes fatal if it carries you over a band of rock. The slide does not have to bury you at all.

Flat runouts and benches. Where a slope steepens then flattens abruptly, moving snow decelerates and piles deep. Debris depth on a flat bench below a start zone is often several times what it is on the slope itself.

Crevasses and moats. Debris fills them, and so does anything travelling with the debris.

Rocks and stumps. Blunt trauma at speed, and pinning.

Water. Creeks, lakes, open water below a slope. Burial in slush, or immersion.

The consequence question

Once you start thinking this way, the assessment changes shape. Instead of asking only "will this slide", you ask two questions:

How likely is it to release? — from the forecast, the snowpack, recent weather, observations, slope angle and aspect.

What happens if it does? — from terrain traps, slope size, and where the debris ends up.

A slope with meaningful likelihood and low consequence — small, open, mellow runout, no traps — may be an acceptable risk. The same likelihood above a gully choked with trees is not.

This is also why experienced travellers happily ski certain steep slopes on marginal days and refuse other, gentler slopes. From outside it looks inconsistent. It is not: they are pricing consequence, not just probability.

Where people get caught

Terrain traps punish exactly the places groups naturally choose. Gullies are efficient travel corridors and often hold the best snow. Benches below slopes are convenient regrouping spots. Treed slopes feel protected and are popular in poor visibility.

The classic incident is a group stopping to regroup in a sheltered spot at the bottom of a slope, which is the runout, with the whole face above them. Sheltered from wind is not sheltered from snow.

Choosing better ground

Prefer ridges and broad convex rolls over gullies and depressions. Prefer open slopes with clean, wide runouts over anything that funnels. Regroup on ridges, on islands of dense timber, or well out of the runout — measured generously, because runouts extend further than they look, especially in a bad year.

Look up before you stop. It sounds obvious and it is skipped constantly. The question "what is above me right now" should be as automatic as checking for traffic.

Travel one at a time through any exposed section, with the others watching from a genuinely safe spot. One at a time reduces the number of people caught and guarantees there are witnesses who saw where you went — which, as anyone who has run a companion rescue knows, is worth more than almost anything else.

The mental shift

The most useful habit is to stop looking at slopes in isolation and start looking at systems: the start zone, the path, the runout, and everything connected to it.

Read the whole thing on the way in. Note the traps. Then, when you are tired on the way out and light is fading and someone suggests the direct line down the gully, you already know what is in it.