Ed it is my understanding that 96% of the spawn takes place in the aformentioned three. 90% in the Chesapeake. Also in the Ace system in South Carolina (Congaree river) and the California Sacramento and San Joaquin rivers.
As long as the river is 32 miles long and twice as long during heavy precip year they can and do spawn other rivers and even some conjecture about landlocked tailrace water systems.
How inappropriate to call this planet Earth when it is quite clearly Ocean.
Arthur C. Clarke