What about signature Hoover Dam? Is it the same as iconic Hoover Dam?
'Signature' refers to something typical of one person; many TV presenters have a 'signature tune'; when you hear it, you think of them. There is something visual about something that's iconic - think of the derivation from church images - Gk eikon. Online Etymology Dictionary
(Of course, 'iconic' has moved on from its Greek Orthodox church origins, and some people - especially in advertisements - use it to mean something vaguely like 'jolly important [buy this]'!)