How did freshwater fish species spread from one drainage basin (river system) to another?

o as eggs on bird feet?

o otherwise carried by birds?

o squirmed across land (there are a couple of species of fish that actually do this)?

o swam from one to the other during a time of flood?

o carried by people (including "stocked")

o the land has shifted in recent millenia. At one or more times those rivers shared a common source (possibly ice-age repeated inundation).

Stream capture, flooding, and trips through saline water from river mouth to river mouth

Mostly through floods. Some fish can also move over dry land. Rivers will also change courses and they may join and disjoin.