My question borders on suppose we are back 400 years ago and know almost
nothing
about modern paddling techniques. Maybe I'm an indian (American) on a river in
the midwest. The tribe has noticed something about the way water flows. Perhaps
it's on a river the tribe has never seen before. How do I turn that into a
response while paddling?

If you were an Indian not knowing "modern techniques" I'm sure you'd know
"ancient techniques" that were every bit as effective! The point is
"experience". You come to a bend in the river that you've never seen before,
and you have a good idea of how to handle it because if you haven't seen that
particular bend, you've seen plenty of other bends and they all had something
in common--what we'd call "physics".

Perhaps another way of thinking about this is to assume you are blindfolded and
paddling down the river, and you are with a companion. He says, "Small
eddy dead
ahead." What do you do to prepare for it?

As another example, maybe more realistic, suppose you are given some sort of
waterflow map of a river that you have never been on. How do you prepare
for it,
tactically or technique wise, without ever having been on it and only knowing
from the map of a written description what to expect?

Once again, experience is the key. Based on a verbal description or a map, you
know what to expect when you get there and how to deal with it. You could say
that you "speak the language" that allows you to turn the description into
a realistic plan of action. It would be just like a description of anything
that wasn't right there in front of you but you had a basic familiarity with
that kind of situation--you'd know what it was likely to be in reality.

In fact this kind of thing does come up in whitewater canoeing: you don't get
an intimate look at every drop before you go through, but someone may give
you a quick description ahead of time. "Head right just after you get round
the first bend, then watch for the main stream rejoining on the left. Then
there's a choice of gaps between the big rocks--just left of center is best
in this kind of water, and you can make a quick run there, or set over to it.
There'll be some big standing waves on the run-out, and you may pick up some
water. There's a nice big eddy on the left at the bottom and we'll meet there
and anyone who needs to empty their boat can do it there."

Stuff like that.