so maybe I phrased it poorly. You are right the appending needs to be done to make a Vector2 for the UV input in the texture sample.
What I mean by that is that in the talk there are different "calculations" to get to the base and it looks like you tried to apply both of them.
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Both are given in the lecture and give the same result. However, these are not the smooth gradients that appear in the video. These result in UV coordinates that are pretty drastic and result in stretched textures. They should also have the same "cost" benefit.
In the end, it's a calculated river map. I haven't done this yet, but I use texture-based river maps quite a bit. (A great unreal tutorial from Simon: https://www.youtube.com/watch?v=tEr3NE_XLbc).
You could just make a flow map and do it with it, but because of the texture search, the cost is higher than a math approach like the one above.