[8bitsrule]

Interesting to consider why they might have wound up at Cooper's on the Salmon River

https://en.wikipedia.org/wiki/Salmon_River_(Idaho)

instead of going farther up the Columbia. Might not have been a viable option!

https://en.wikipedia.org/wiki/Cordilleran_Ice_Sheet

"On the western edge at the Haida Gwaii ... the lower thickness of the ice sheet meant that sea levels were as much as 170 metres lower than they are today.... migrants ... were able to travel southward during the deglaciation process due purely to the exposure of submerged land between the mainland and numerous continental islands...."

[boomboomsubban]

Or they did go further up the Columbia and we haven't discovered evidence of it. It's not easy for things to survive for fifteen thousand years.

[undersuit]

Nor would the timing of the last predicted flood of Glacial Lake Missoula help.

[throwaway5752]

Particularly on the Columbia River: https://en.wikipedia.org/wiki/Channeled_Scablands