Stone,

I was relating it to the first sentence of the reply, with the part about college magnifying the point. It's not so much the not knowing about it before he studied it; it's that in his late 20's he was amazed that it could be a part of someone's general knowledge, without them being taught.

Alluvium is eroded material from one place, deposited by water in another place. I use the word pretty frequently to describe a landscape, as do others I know. Valleys fill with alluvium from the mountains that surround them. Just 35 miles east of me in the desert at the base of tall mountains are huge alluvial fans, where water flows down canyons, carrying sediment, which then spreads out at the bottom. So it's common among anyone who knows the countryside to know what it means.
River deltas are composed of alluvium, but are not the only type of alluvial feature.