I'm surprised they had time for this during WWII

Wow. I love this image.

Would be less interesting for (most) of the Snake, Columbia b/c both are more constrained by geology (rock canyons) that prohibits millenia-scale meanders across a wide alluvial flood plain (MS delta). Need a much longer time scale to see changes in those rivers, like the Snake River capture event from either the Klamath or Humboldt into the Columbia. Can trace historic channels for other NW rivers with alluvial floodplains or deltas, though - think Skagit Delta or mouth of the Columbia.

the Mississippi would have permanently changed course decades ago to exit to the sea via the Atchafalaya swamp except that would have left New Orleans dry - ironic? - and so the Corps of Engineers has been spending wads of money to keep it in is "current" channel. John McPhee wrote a lovely essay on this a few years back

this is so sad to me bc now we don't let it meander. we are killing the river.

RIght, gorges and whatnot. You'd think since I grew up around the Snake I'd know that. Ah, well. I can still moon over a pretty map.

Yes,
Rivers age, as do most things, and this type of pattern could only be made in very old aged rivers. Also the Snake is a river that cut the deep channels because of the uplift of the strata much like the Colorado. The canyons didn't get deeper, the land rose allowing for the erosion to make it appear that the river cut downwards. Referred to as an antecedent stream in geologist's terms.

@1: The real war construction work was being done by the Navy Seabees in the Pacific Theater, and by the Army Corps of Engineers at Los Alamos. The guy who drew this was just frustrated he didn't get picked to work on the Manhattan Project, so he took it out on the Mississippi, LOL.