Susquehanna River Valley

From my Hydro Prof. @ Bloomsburg:
"There is no "rule." Generally, when a relatively large tributary joins a main stream, the main stem is the one that has more water, the "branch" is the smaller of the two. It's a branch probably because even though it's smaller, it's still pretty big!

Naming is often a cultural/historical thing, so you have to consider the history of the river and how it was first explored by Europeans. They came up the river, so their first encounter with the branch was at the confluence, where is was clearly a branch and not a separate river. The "branch" name also makes it clear how these rivers connect, and that's an important idea in navigation.
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Basically, a name is a name, and you can't read anything into it about the physical name of the system. (Although in this case, they really aren't two separate rivers in any real way, from a water use, hydrology, pollution, ecology perspective.)"

Why does Pittsburgh have three rivers coming together, instead of a branch flowing into the main river. The Pittsburgh 3 rivers junction looks the same as the Sunbury two-branches of the same river juntion to me. What gives?