So, I had always played the game as though events were (unless otherwise stated), one shot affairs.

However, reading the rules it would appear that there is nothing explicitly stated as to them getting discarded after the effects have taken place. I might have just missed a sentence though, if I have, could you please point it out to me?

"Types of adventure cards:

Now, most do have on them some variation of "... and then discard this card.", which is crystal clear. Others however, have got me thinking:

Tumbling Boulder (Highlands):

Event

Roll 1 die:

1) Crushed, lose 1 life then discard this card.

2-6) Move the Boulder 6 spaces counter-clockwise. For each space the Boulder moves onto, any characters there lose 1 life and any cards are discarded.

[it seems simple enough, but if you roll a 2-6, does the Boulder move backwards, and, should players continue to keep landing on it, continue tumbling backwards until a player rolls a 1?]

Ragnarok (base set):

Event

The final destiny of the gods has arrived! Discard all faceup Enemies, Places, and Strangers in all Regions.

[There is no mention of this card getting discarded on the text of this card. Could that mean it remains in play, getting "re-triggered" every time someone lands on the spot? That would make some degree of sense actually, in terms of what the card is representing. It's not just a one time event, the world is in constant conflict...]

Sliding Walls (Dungeon):

Event

The ceiling and walls start to close in on you. Roll 2 dice, if the score is equal to or less than your Strength, you are safe. If it is higher, you lose 1 life.

[Another event with no mention of it being discarded after being encountered. On one hand, this makes sense that it would remain in play, it's a trap in the dungeon. On the other hand, wouldn't it be a place if that were the case (see Snake Pit)? I'm not sure. I could see it not being a place because there are cards that specifically interact with those, and this isn't really a "place", just a trap in a room.]

Lone Dwarf (Dungeon):

Event

Roll 1 die to determine how he reacts to you:

1) Attacks you with Strength 4

2) Move back 2 spaces

3) Lose 1 gold

4) Gain 1 gold

5) Move forward 2 spaces

6) Joins you as a Follower; add 2 to your result at the Treasure Chamber

[This guy is similar to the Sliding Walls card above, in that he seems that he would be better represented as a Stranger, not an event if he is supposed to stick around, but there is no mention of the word discard anywhere on him. On the other hand though, since there are negative effects that can take place, it would make sense that he is an "event". Especially if you take him then ditch him somewhere. You come across a mad or sketched out dwarf, and he reacts unexpectedly. Also, the wording is... funky when dealing with this card. As I understand it, if you roll a 2 or a 5, the character is supposed to move, but since it's to determine how HE reacts to you, wouldn't it be him that should be moving around the board? It could just be him coming at you from an unexpected angle and beating you backwards (or forwards).]

Edited September 7, 2013 by feffg

Events are usually discarded after they're resolved, if not stated otherwise. In some cards, like the ones you quoted, there's no discard condition and the Rulebook doesn't help, so a general clarification by FFG would be welcome.

As for Events that last many turns, the FAQ added this explaination:

3. Any Events in the space must be encountered. Note that an
Event that lasts more than one turn remains on the space
as a reminder, and still counts as a card on that space. If
a character encounters the Event again, the effect does not
“reset” and last for additional turns.

Events get discarded once the effect gets resolved. They don't instruct you to discard them because that rule applies to all events. The exception is when the event lasts for multiple turns or rounds. That is when the text will say otherwise, because it is an exception rather than the norm. In those cases, they remain on the space for that many number of turns. An example is the Blizzard.

All the cards you mentioned would get discarded as soon as they are resolved. The rolling boulder would not remain as a permanent structure on the board.