Here's another inspiration from the Maryland convention after seeing the Fairfax Stone headwater spring of the Potomac near Backbone Mountain.
I have posted a page devoted the headwaters of U.S. Rivers more than 350 miles long.
Its base data is infoplease. But as usual it will need to be tweaked (the encyclopedia placed the headwaters of the Potomac in Garrett County, Maryland).
I was disappointed the Hudson with its home on Marcy didn't meet the 350 mile requirement. Of course there's always the Arkansas with its home near Elbert.