...if this was the only condition for "what city becomes a capital." Looking at Germany, there's Hamburg, Köln, München...all on nice rivers, but none of them were the capital of Prussia when Germany was first unified - since Prussia was the essential driver of that political process, Berlin became the capital. Something of a coincidence in the grand scheme of things, really.

For some reason, it's usually a faraway landlocked province that triggers unification. Not the rich and coastal ones.

Turin-Savoy in Italy, Prussia in Germany.