IIRC the las one was what made our plains states so fertile. The heavy ash fallout went nearly to the Mississippi and that is farther away than 350 miles.
It would be spectacular and would make half the US super fertile.. after it was rained on for a half a century or so. Yeah it would probably kill all the fish in the rivers to the east of it, but eventually it would be better.
The earth and nature are fantastic when viewed on time scales we can only imagine.
The ash was six feet deep in Nebraska.