He says the correct pronunciation of ‘Arkansas’ is ‘Ark-an-saw’ which is correct for the State but not necessarily for the river. I know in Colorado it is ‘Ar-Kansas’ river.
It is pronounced that was in Kansas too.
Must depend on where you live in the state. I’ve lived in northern CO for 48 years and have never heard it pronounced as anything but Ark-an-saw.
There is a town in south central Kansas named “Arkansas City.” God help you if you pronounce it like the state of Arkansas. The locals call it “Ar KAN sas” City, with the final s sounded. The Arkansas River (pronounced like the state - no final s sounded) runs past it.
(Parenthetically, FWIW, Obama’s real father, Frank Davis, is from there.)