Why does the Parks Highway from Nenana to Fairbanks Alaska, stay high on the ridges?

instead of staying along the Tanana River? Is it due to construction problems with permafrost?Why does the Parks Highway from Nenana to Fairbanks Alaska, stay high on the ridges?
I'd think economics would be the main reason it's built high, and permafrost, with the extra expense in road construction (and the resulting after-construction thermokarst problems) is the biggest reason it's cheaper to build where they did. Though constructing along an active river cut may alleviate some permafrost issues, flood threat and erosion would probably outweigh that advantage. Also, depending on the specific section, landslides would be significant factors as well. Many sections of that highway still were built on some sort of permafrost I imagine, it was just cheaper to build it higher.


Plus it gives the bazillion tourists nicer views :-)