The beauty of digital is your not limited to 36 exposures so NEVER wipe images off cards until your in doors at home, in fact never wipe them off full stop until there in Lightroom* (*other editing packages are available)
In fact the only time I delete files off a card is when shooting events work and I've gone through a 8 gig and 5 4 gigs and have to rotate the cards again. (Not filling them all obviously
) just changing between rings, events types etc....
With hindsight you could argue the waterfall ain't going to move so if you know their maybe wild life around best keep the big lens on at all times until you get settled by your waterfall.
In my out and about kit my 120-400 will only go in one spot in my camera bag with the camera body on it so it's always on the camera as I see it if I'm going to use one of the smaller lenses (for landscape etc) I'm going to have the time to change.
Better luck next time and we all learn by our errors thats why the next time you'll shoot that great, very special image
