Thanks Tommi, thanks Voriig, pretty good input!
So you also use Flows on gound level, I never had the idea of a ground level jet stream, I used more Voriig interpretation of mist or, in my case, something like a heat reflection, some hinderance starting soft and becoming in few steps really solid.
I´m not sure if you know the old Jaiman Flow Chart, there are a lot of flows put in with the forge in Arion etc. This was the reason why I had to give my players after years of years of playing some encounter.
I pretty like the idea of people trying to make money out of digging tunnels under stable flows!! In my own campaign my players have the option to use the skill power projection to pass a essaence flow, but even the easiest test is already a sheer folly, and becoming harder with more energy of the flow or trying to let more people pass etc