How can I tell if a natural snow-bridge is safe to cross?

In early summer in the high mountains, snowfields may lie over rivers. These rivers may be otherwise tricky to ford, and it can be very tempting to use the snowfield to cross the river. However, this can, of course, be risky. What is a good way to test whether it is safe to cross? I've heard of using a stick to push the snow, but I wonder if that mightn't collapse the whole snowbridge with me on it...

Báhkkabahoknjira under the Báhkkabahokčohkka, near Abisko, Sweden, 22 June 2013. Thick enough to cross (although a bit tricky because of the steepness).

Njulkkostakjohka above Alisjávri, near Abisko, Sweden, 6 June 2013. Rather not cross this one.

I will not recommend crossing if the bridge is only composed of frozen snow because snow does not support a lot of weight. You should check the following:

• The ice should be at least 15 cm thick (be careful to differentiate the frozen snow from the clear ice, the 15 cm applies only to clear ice)
• The bridge should not contain any water on it (sign of melting or possible weak structure)
• The temperature for the last days must be at least less than freezing (to prevent any melting)

Furthermore, you should:

• Avoid places where the current is strong (because they weaken the thickness and the structure)
• Avoid places where the bridge is longer (Ice is thicker near the extremities and thinner in the middle)

Last but not least, the following measures are used for lake crossing:

• 10 cm for people
• 12 cm for snowmobile
• 20 cm to 30 cm for small cars
• 30 cm to 38 cm for trucks

References:

• Some snowfields remain until August even if temperatures rise above zero from May, so surely it can be safe to cross even if the temperature for the last days was not less than zero? Why would there be clear ice in a snow-field if not from a spring freeze-melt cycle? And how can I tell apart ice from frozen snow, in particular when all is below a surface that looks like snow? Jul 30, 2013 at 16:56
• Sorry I was not clear but what I mean is that lower temperature drastically decreases the power of ice to withstand your weight. For ice formation, well it depends on many factors (the weather, the river shape). Natural bridges can be created naturally but it is rare on larger river. Last but not least, if you are not able to differentiate between the different layers, it means that you are dealing with only frozen ice. I suspect that the stick you are talking about is used to detect either the ice depth, the snow depth or how firm the snow is. Jul 30, 2013 at 18:16

If you have no choice but to cross a snow/ice bridge then normal practice is to be roped in with two other people and to use a snow probe. If a 3m probe passes through without resistance then it's not safe to cross.

Normally the 3rd will self belay the leader and second, who start about 10 feet back, with the second belaying the leader. Cross in a perpendicular manner, and distribute your weight as much as possible (skis, snowshoes). Whatever you do, don't stomp on the snow.