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Where is the better chance the ice on the frozen river in winter will be thicker and safer to cross?

I expect the river width oscillates between the following conditions:

  1. on a shallow, wider, slower stream with a surface rippled by rocks below
  2. or a deeper, narrower, faster current with a calm surface?
  3. something in between? Where?

Why is it the best? How can I find that spot from a map?

Example situation:

Example map

But the question is rather about principles: What conditions are best to look for?

A detail from the top of the map to judge the water surface:

A river seen from a satellite

It is handy to understand these principles to plan a trip in advance, when there will be a river crossing needed, and one is not sure whether it is cold enough, so the ice is thick enough everywhere.

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  • I added a situation to be more specific, but the question is rather on principles what to look for. Feb 5, 2023 at 22:25
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    You seem to have the conditions in 1 and 2 muddled. A shallow, wider, slower stream is more likely to be calm, and a deep fast river is likely to be rough. Exactly where it will freeze, and how thickly, might be subject to many local conditions. I would say you are best not to attempt crossing a frozen river without an expert guide. Feb 5, 2023 at 23:14
  • With so little vegetation at such elevations, this would seem to be in far northern Lappi?
    – gerrit
    Feb 6, 2023 at 7:55
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    NB: kahlaamo means ford, and a place that's a ford in summer may not be a bad place to cross in winter either.
    – gerrit
    Feb 6, 2023 at 7:58
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    Wherever a spring enters the river, you could get very thin ice next to very thick ice. Feb 6, 2023 at 12:06

5 Answers 5

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The only factor that is going to be important is the speed of the river. Faster parts of a river stay liquid longer, and are going to give you a thinner surface. A surface broken by rocks helps very slightly.

However this is only going to help you slightly. You need about a 15cm ice thickness to traverse it safely. If still parts of the river are 15cm then faster parts are not going to be much less. If there is any open water then it's not really safe.

The only other factor is the depth of the river, and only in the sense that if you know the river is shallow enough to wade in then you won't drown if you fall through the ice, provided you have the right waterproof equipment. Getting yourself wet in freezing conditions can be as deadly as drowning.

In short the ice strength and coverage is going to be hard to predict from geographical features, even more so from maps. There are calculations for likely thickness of ice based on recent temperatures but they are for still lakes. River ice will be thinner. Similar articles will give you other advice, which I won't bother to repeat here.

Be aware the ice is treacherous, and you should only be walking on it if you are sure that you have a good thickness.

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    In this case there's a decent chance of freezing to enough depth - the text on the map is in Finnish where freshwater freezes enough to do stuff on. But river levels can drop leaving a thin layer of ice, above air, then water, bringing another risk that's not obvious.
    – Chris H
    Feb 6, 2023 at 7:07
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    So if you want slow moving waters you want a large cross section of the river, so a spot where the river is both very wide and very deep?
    – quarague
    Feb 6, 2023 at 8:38
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    +1 for most of this, except “If still parts of the river are 15cm then faster parts are not going to be much less.” — in my experience (Sweden and Northern US) I’ve often seen rivers/lakes that are solidly frozen in some stretches but much more thinly-frozen (or even open) on other parts quite nearby. Feb 6, 2023 at 9:19
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    I assume you meant faster parts give you a worse / weaker / thinner frozen surface (and not a better one)?
    – NotThatGuy
    Feb 6, 2023 at 12:54
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    There is also the third factor, which is sometimes absent, but sometimes present to various extent: the plants growing in the river. The ice is generally much less strong where the plants are frozen into it, especially near the spring when the sun starts to shine. You definitely want to walk straingt from hard ground and onto hard ice not crossing those scirpus-covered areas, especially if the strength of the ice is in any doubt. Feb 8, 2023 at 6:55
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There is no general rule in winter, it simply depends too much on conditions.

  • Most rivers have a lot less flow in winter, as precipitation falling as snow will not run off into the river.
  • Rocks in a river are often glaciated with ice, which makes them a bad helper for crossing
  • Not only the thickness of ice is relevant, but also the amount and condition of snow on top
  • Often one can also find a snow slide building a bridge across smaller streams

And do not forget, not only can crossing the river itself be a challenge, but also descending to the river bank. This is just as important for consideration as the river crossing. Unfortunately, even high resolution maps will not be show micro features that might be relevant here

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safer to cross

Hope for the best, plan for the worst: think ahead what you will do if the ice breaks.

In this case you can cross at the "Kahlaamo" (ford, crossing place) near the bottom of the map. It will be shallow enough and enough rocks that you can minimize the danger of drowning. Then it becomes just a matter of dealing with wet clothes, which you can prepare for by having something dry to change to.

If there isn't a shallow enough place, you could evaluate the risk of going across with safety rope attached. Remember that the current can pull you below the ice.

There are probably ways to figure out where the ice will be thickest, but even then it might not be thick enough. So instead figure out where and how you won't die if you fall through.

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In a meander the fastest flow and the deepest water are on the outside of a bend. The outside of a bend is therefore where you're most likely to have trouble with both thin ice and deep water.

If you have to cross on a bend, it makes sense to cross outside-to-inside, setting up safety cover if possible for the first stretch. This is because getting someone out of the water is easier from a bank than from on top of ice - and safer. It also mean that if you have to retreat across marginal ice you don't have to go as far. Personally I'd have a rope ready on the bank, possibly a handline too (a rope to hold; it can be run half a turn round a tree - if you can find a sturdy one - or rock, and you can hold both strands, then it can be recovered).

The aerial imagery in the question is for the top left area I've highlighted on the map here:

map

If you were going to cross from south to north in that area, my red line might be good:

OP's aerial view

The lighter, browner water suggests it's shallower, and the deepest bit is close to the south bank where there are trees to tie off a rope if needed for rescue. It wouldn't be so good coming from the north

The southerly highlighted box has a ford ("Kahlaamo") marked aerial view further south (person icon and my red line). This is interesting as it's not the widest part of the river but does look shallow. It's just downstream some rapids and a pool (the river flows south to north). It may have a bit much of a flow, if it's neither deep nor very wide.

Further upstream there's what looks like a very shallow gravelly bit (red line again). This would be interesting, though the increased flow around the rocks (see the small bits of white) might not be good in winter.

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  • The river can't be shallow in a place where it is literally like one third of the width of another place you call shallow... All that water has to go somewhere, so the river is either wide and shallow, or narrow and deep. If the narrow parts are fordable, then it's really just a very small river and you don't have to worry about too many things anyway.
    – fgysin
    Feb 8, 2023 at 7:49
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    @fgysin it can if the flow speed increases because of a gradient. In a basically flat river, you don't need much gradient to enable that. This effect is quite common on the rivers I kayak.
    – Chris H
    Feb 8, 2023 at 8:40
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There really isn't one way only from a map as there are features that can affect were you are going to cross that the topo map won't be the most accurate about or will be misleading. Satellite imagery along with the topo can give a better picture of the type of terrain that you are dealing with.

Looking at your map in the question, I would probably look near the north edge of the map. But I would need to know more about the area. It doesn't look too wooded but are the flat areas near the river dry, weedy, or full of thin willows that are hard to get through.

Even with winter, I would probably want to find a deadfall across the river which obviously wouldn't be on a map.
As pointed out in the comments, crossing a frozen river carries some risk. So there is also the reason that you are wanting to cross, could you avoid crossing at all? The map in your question has a couple of trails that parallel each other on the river, could there be a marked crossing further north?

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  • We've now gained the coordinates, and a translation of Kahlaamo (= ford), so this might be the best crossing point
    – Chris H
    Feb 6, 2023 at 14:49

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