Longest routes following a continous named waterway

[Jardine]

US 385.




(Ogallala Aquifer)

[webfil]

Clever.

[DTComposer]

I-5 in California follows the Sacramento River from Sacramento to Mt. Shasta

I don't know if I'd count this one - although they parallel each other, the river isn't visible from I-5 between the Sacramento Airport and Red Bluff, and they are often over 10 miles from each other.