Too complex metaphor for me. If I've understood you, the flow is not exactly the trade. That's open to a lot of distracting objections. Beautiful anyway :)

Thank you. Maybe it's easier to recover the river from the optimisation case: we profile first, then optimise. This is because the only things that count are what actually eat wall clock time. Similarly, the only places in value chains that accumulate wealth are where there are actually consistent margins. And it doesn't matter if there are rocks in a riverbed (which could potentially make bottlenecks or waterfalls), if they're located in a pool between higher rocks, so the water level is higher than they are.

Does that make more sense?

I didn't mean it doesn't.

My point was that trade, in a wide sense, creates wealth. No one would cross the sea or the desert if there wasn't a prize, so I take for granted that there is an inbalance.

"Bottleneck" seems to suggest that commerce is like a natural process. It isn't. There are trade cultures like Phoenicians or Renaissance Venice, but often the powerful don't like commerce. Commerce implies some values that subvert authoritarian regimes, maybe just because different ways of understanding life make contact.

So when I hear bottleneck, I think of trade restrictions, not flow. You mean bottleneck as a funnel where the flow is more intense and thus more susceptible to capture wealth. It makes sense, but it's somehow a twisted metaphor, because it's the inbalance (gravity, or demand + the cheaper production elsewhere) that creates the flow, the bottleneck just make it easier to exploit. But a cascade can be wide or narrow, tall or short. In any case, there's water falling...