Actually, adding water can CAUSE an eruption if hot magma is present; generates steam, and steam is what propels most eruptions.

Yellowstone always comes up as The End Of The World As We Know It. And it would be, but it's very unlikely to happen without plenty of notice.

Smaller eruptions have caused localized but significant ashfall damage. Lassen's ash in 1915 fell as much as 200 miles east, and did affect train traffic in Nevada. One could expect BNSF Inside Gateway and the former WP in western Nevada to be strongly affected should a similar eruption happen again, which it almost certainly will from some vent in the Lassen region. But in the great scheme of things the most likely eruption at Lassen would be small compared to the ones that get most of the news.

Of course there was St Helens in 1980: also a relatively small eruption on the Volcanic Explosivity Index (VEI), but significant. Anybody around here have experience trying to operate trains (or even just survive) in eastern Washington/Idaho/Montana back then? Both St Helens and Lassen have produced much larger eruptions in the distant past than they have (geologically) recently.

Here's another combination-of-big-eruptions map, from USGS:


"Novarupta" in that graphic, btw, is in the middle of nowhere in Alaska; it's next to Mt. Katmai, and the whole place is now a National Park. It filled a large valley with super-hot ash, creating the "Valley of Ten Thousand Smokes." It *was* a big one; nowhere near Yellowstone, but much bigger than St Helens.

And the old standard what's-erupted-when graphic from USGS:


Alaska has many more eruptions than the western US otherwise. Makes sense: Pacific plate pushes under Alaska much faster than things are happening in the Cascades & California/Nevada/Idaho/Yellowstone generally. One eruption in Alaska, before such things were understood better, flamed out all 4 engines of a 747 passing through some miles downwind; luckily, they DO glide (though poorly) and that one did long enough to get a couple of engines restarted and make an emergency landing in Anchorage.

If you want to explore volcanoes some more, check out the Smithsonian: [volcano.si.edu] and USGS: [volcanoes.usgs.gov]